15. You obtain the durations of 3 sequential events using three different timers, as
1.63 seconds, 2.6 seconds and 3.558 seconds respectively. The combined duration of
these events is the sum of these three times. This sum should be reported as:
a. 7.7 seconds
b. 7.788 seconds
c. 7.79 seconds
d. 7.8 seconds

Mastering Significant Figures in Addition for Timer Measurements: CSIR NET Physics MCQ Solved

The correct answer is b. 7.788 seconds. When adding measurements like 1.63 s, 2.6 s, and 3.558 s, the sum 7.788 s matches the precision of the least precise value.​

Addition Rule Explained

For addition or subtraction of measurements, report the result to the same decimal place as the least precise number involved. Here, 1.63 has two decimal places, 2.6 has one, and 3.558 has three—the limiting factor is 2.6 at one decimal place (tenths). Adding gives 1.63 + 2.6 + 3.558 = 7.788, kept as is since it aligns to the tenths place without further rounding needed beyond the calculation.​

Option Analysis

• a. 7.7 seconds: Too imprecise; ignores digits beyond tenths despite 2.6’s precision limit.​

• b. 7.788 seconds: Correct; retains exact sum to the tenths place per rules.​

• c. 7.79 seconds: Rounds up incorrectly; 7.788’s hundredths digit (8) needs no rounding to tenths.​

• d. 7.8 seconds: Over-rounds to units; violates precision retention.​