Question

A long distance runner does a first lap around a track in exactly 50 seconds. as she tires, each subsequent lap takes 20% longer than the previous one. how long does she take to run 3 laps?

Answer 1

new percentage=100+20=120%
1st lap= 120%of 50= 60
2nd lap= 120% of 60= 72
3rd lap= 120% of 72= 86.4 second

Answer 2

The first lap takes 50 seconds. Each subsequent lap takes 20% longer than the previous one.

So we can write the time taken to run the $n$the lap is $T_n=(1+\frac{20}{100})T_{n-1}= 1.2T_{n-1},n=2,3,4,...$.

So we can write, $T_1=50\\ T_2=1.2T_1=1.2*50=60\\ T_3=1.2T_2=1.2*60=72$

So the time taken to run 3 laps is

$50+60+72=182\;seconds$