E ExamMaster

CBSE Class 5 · Mathematics

Racing Seconds

Official NCERT chapter from Math-Mela (book code eemm1). ExamMaster notes are original teaching at CBSE Class 5 depth.

This lesson follows the official chapter “Racing Seconds” in Math-Mela. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 5
  • Easy level
  • 8 concepts

1Start time and finish time give a duration

Duration is finish minus start on the same time-track. 4:10 to 4:25 is 15 minutes.

4:25 is a clock-time, not the duration.

Figure. Duration is the span from start to finish, not the finish clock-word alone. From 9:10 to 9:25 the span is 15 minutes.

How it works

  1. Write start4:10.
  2. Write finish4:25.
  3. Subtract15 minutes.

2Minutes and seconds sit on a racing clock

60 seconds make 1 minute. A race time 2 min 15 s is 135 s if you need seconds only.

Writing 2:15 without saying min:s can be confused with a clock hour.

Figure. A racing minute is a bundle of sixty seconds. Write both units. A clock face is not needed: the equality 1 min = 60 s is the fact.

How it works

  1. Name the unitsmin and s.
  2. Use 60 s = 1 min if they must meetThe rate.
  3. Keep clock-hours out unless the question is clock-timeTwo tracks.

3Twelve-hour and twenty-four-hour writings name the same moment

3:00 in the afternoon is 15:00. Same moment, new writing.

3:00 and 15:00 are not 12 hours apart if they name the same afternoon.

Figure. 3:00 pm and 15:00 name the same afternoon moment. Add 12 after noon to move from the twelve-hour writing to the twenty-four-hour writing.

How it works

  1. Ask morning or afternoonThe half of the day.
  2. Rewrite 12-hour to 24-hour if askedAdd 12 after noon, except 12 itself.
  3. Check they point to one momentSame instant.

4Morning and afternoon split the day

a.m. is before noon. p.m. is after noon. A duration can cross noon: 11:50 a.m. to 12:10 p.m. is 20 minutes.

Forgetting the split makes 11 to 12 look like 1 hour the wrong way.

Figure. The day splits at noon. Morning writings use am; afternoon writings use pm. Noon is the cut, not a third half.

How it works

  1. Name the splita.m. or p.m., or 24-hour.
  2. If the span crosses noon, count through 12:00Do not subtract blindly.
  3. Write the durationHow long.

5Compare who took longer

Longer is the larger duration after both are in one unit. 2 min 10 s versus 125 s: rewrite, then compare.

A louder cheer is not longer.

Figure. Longer bar is more time, so that runner was slower. Dev’s 55 s bar is the longest; Ria’s 42 s bar is the shortest.

How it works

  1. Rewrite both durations in one unitSeconds, or min-s.
  2. Compare the numbersLarger is longer.
  3. Name whoThe larger duration.

Who took longer

Ria 2 min 10 s. Dev 125 s. Who took longer?

  • Ria in s2×60+10=130 s
  • Dev125 s
  • LongerRia by 5 s

Pro tip. Match units, then compare. 2:10 is not 210 seconds.

6Elapsed time is a subtraction of times

Elapsed means how long passed. It is a subtraction on the clock-track, not a date-box.

A calendar yesterday is not this elapsed-time.

Figure. Elapsed time subtracts the start minutes from the finish minutes when the hour matches. 45 take 20 is 25 minutes. Crossing an hour needs a regroup, not a new story.

How it works

  1. Mark start and finishTwo clock-times.
  2. Subtract along the trackBorrow 60 minutes if needed.
  3. Name the spanElapsed.

7Match the two writings of one clock

7:00 p.m. and 19:00 must pair. A mismatch is two moments or a rewrite error.

Do not add 7 + 19.

Figure. 7:00 am and 07:00 are two writings of one morning moment. Matching them is a pairing, not a duration.

How it works

  1. Take the 12-hour time7:00 p.m.
  2. Rewrite to 24-hour19:00.
  3. Demand they name one momentOr fix the rewrite.

8Yoga is the story; the duration is the maths

A yoga session is the picture. Start, finish, and elapsed are the maths.

Do not invent a yoga-only minus.

Figure. The story can be yoga, a race, or a bus. The mathematics is the 20-minute span from 7:00 to 7:20. Name the duration; do not count the story-word.

How it works

  1. Keep the settingA session.
  2. Compute elapsedFinish − start.
  3. Use ordinary time-subtractThe mat is not a new rule.
4:10 to 4:25 is
  1. A duration of 15 minutes
  2. The clock-time 4:25 as a length
  3. 125 seconds always

Duration is the span, not the finish stamp.

Notes

  • The official chapter title is “Racing Seconds”. Teach the school test for that title, not a contest shortcut.
  • If a step needs a later class, stop. The next official chapter will pick it up.

Formulas

  • Duration = finish - start (same track).
  • 60 s = 1 min.
  • p.m. hour h (not 12) \rightarrow h+12 in 24-hour writing.

Recap

Hold these pegs from the official chapter “Racing Seconds”. The wording is ExamMaster’s teaching, not a textbook recap.

Start time and finish time give a duration
Duration is finish minus start on the same time-track.
Minutes and seconds sit on a racing clock
60 seconds make 1 minute. A race time 2 min 15 s is 135 s if you need seconds only.
Twelve-hour and twenty-four-hour writings name the same moment
3:00 in the afternoon is 15:00. Same moment, new writing.
Morning and afternoon split the day
a.m. is before noon. p.m. is after noon. A duration can cross noon: 11:50 a.m. to 12:10 p.m. is 20 minutes.
Compare who took longer
Longer is the larger duration after both are in one unit.
Elapsed time is a subtraction of times
Elapsed means how long passed. It is a subtraction on the clock-track, not a date-box.

Practise Racing Seconds

Reading is free and needs no account. Practice, mocks and progress live in the app.

  • A 4-question practice set that ends the chapter
  • 1 quick check with worked explanations
  • Timed mocks scored with the real marking scheme
  • Readiness tracked per topic, kept on your device
Continue with Google — freeNo card, no trial. Works offline once installed.