NCERT Revision Notes for Class 5 Maths Chapter 12: Racing Seconds – Free PDF Download

Chapter 12 of Class 5 Maths, Racing Seconds, builds on earlier clock-reading skills to cover measuring and calculating time precisely – converting between hours, minutes, and seconds, finding elapsed time (duration), and reading timings from races and schedules.

Key Concepts

Units of Time and Conversion

Time is measured using seconds, minutes, and hours: 60 seconds = 1 minute, 60 minutes = 1 hour, 24 hours = 1 day. Converting between these units means multiplying or dividing by 60 (or 24 for days), similar to converting length or weight units by 1,000.

Finding Elapsed Time (Duration)

To find how much time has passed between a start time and an end time, subtract the start time from the end time, converting to a single unit (like minutes) first if needed, or working in stages (hours, then minutes) if crossing an hour boundary.

24-Hour (Railway) Time

The 24-hour clock avoids the need for “a.m.” and “p.m.” by numbering hours from 00:00 (midnight) to 23:59 (just before the next midnight). For example, 3:00 p.m. is written as 15:00 in 24-hour time (add 12 to the p.m. hour, except for 12 p.m. itself, which is 12:00).

Reading Race and Event Timings

Race timings are often recorded in minutes and seconds (or even fractions of a second), and comparing two runners’ times means comparing which time value is smaller (a smaller time means a faster performance).

Calculating with Seconds

When adding times that include seconds, add seconds first; if the total reaches 60 or more, carry over 1 minute and keep the remaining seconds, similar to carrying in addition of numbers.

Worked Examples

Example 1: Converting Minutes to Seconds

Convert 4 minutes 25 seconds to seconds only.
4 × 60 = 240 seconds, plus 25 seconds = 265 seconds.

Example 2: Finding Elapsed Time

A race starts at 10:15 a.m. and ends at 10:48 a.m. How long did it take?
10:48 − 10:15 = 33 minutes.

Example 3: Adding Times With Seconds

Two relay runners take 1 minute 45 seconds and 1 minute 38 seconds. What is their combined time?
Add seconds: 45 + 38 = 83 seconds = 1 minute 23 seconds. Add minutes: 1 + 1 + 1 (carried) = 3 minutes. Total: 3 minutes 23 seconds.

Quick Recap

• 60 seconds = 1 minute, 60 minutes = 1 hour, 24 hours = 1 day.
• Elapsed time = end time − start time, converting units carefully if needed.
• 24-hour time removes the a.m./p.m. label by numbering hours 00:00 to 23:59.
• In race timings, a smaller time value means a faster performance.
• When seconds add up to 60 or more, carry over into an extra minute, just like carrying in number addition.

Common Mistakes to Avoid

• Subtracting times as if they were in base-10 (treating 60 minutes as if it were 100) instead of carrying/borrowing in units of 60.
• Forgetting to add 12 correctly when converting p.m. times to 24-hour format (12 p.m. stays 12:00, not 24:00).
• Misreading a race time and assuming a bigger number means a faster finish, when actually a smaller time means faster.
• Forgetting to carry over a minute when adding seconds that total 60 or more.
• Mixing up units when converting, such as treating 90 seconds as 1.9 minutes instead of the correct 1 minute 30 seconds.

Home Practice Tips

• Use a stopwatch or phone timer to time a simple activity (like running a lap or solving a puzzle) and record the time in minutes and seconds.
• Practise converting a few train or bus departure times between 12-hour and 24-hour formats.
• Calculate how long a favourite TV show or car journey lasts by subtracting the start time from the end time.
• Compare two family members’ times for the same simple task and decide who was faster and by how much.
• Add up two or three short activity durations (like study time in different subjects) to find a total time spent.

Frequently Asked Questions

Q1. Why does time use units of 60 instead of 100 like most other measurements?
Time units come from an ancient counting system based on 60 (used by the Babylonians), which divides evenly into many smaller parts (2, 3, 4, 5, 6, 10, 12, 15, 20, 30), making it convenient historically for dividing an hour or a circle.

Q2. How do I convert a p.m. time into 24-hour format quickly?
Add 12 to the hour for any time from 1 p.m. to 11 p.m. (so 4 p.m. becomes 16:00), keep 12 p.m. as 12:00, and keep all a.m. times the same except that 12 a.m. (midnight) becomes 00:00.

Q3. Why is elapsed time trickier to calculate than a normal subtraction?
Because time doesn’t work in base-10 – borrowing across the hour and minute boundary means borrowing units of 60, not 10, so it needs its own careful method rather than standard column subtraction.

Q4. In a race, why is a smaller recorded time better?
Because the recorded time measures how long it took to finish – less time taken means the runner completed the race faster.

Q5. What’s a simple way to check an elapsed time calculation?
Add the calculated duration back to the start time – if it gives the correct end time, the elapsed time calculation is correct.

Full walkthrough: Chapter 12 Solutions · Extra Questions.

Written by Satish

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