Answer:
10.5 seconds.
Step-by-step explanation:
Remember that this is an estimate. It is meant for you to do in your head. It is not meant for Google or a calculator.
100 meters is 1/2 of 200 meters.
100 meters time is roughly 1/2 that for 200 meters.
21 seconds * 1/2 = 10.5 seconds.
