Question

The wheels on Annie's bicycle are $20$ inches in diameter. If each wheel makes $3$ full revolutions every second, then how many feet does Annie travel in $1$ second? Give your answer as an integer, rounded to the nearest foot.

Answer 1

We have that
diameter=20 in---------------> r=d/2--------> 20/2---------> r=10 in
[length of a circumference]=2*pi*r-------> 2*pi*10-------> 20*pi in

we know that
 1 revolution----------------> 20*pi in
3 revolutions (one second)----------------> X
X=3*20*pi---------> X=60*pi in  ( this is in one second)

1 foot--------------> is equal to 12 in
X-----------------> 60*pi in
X=60*pi/12--------> 5*pi ft------------> 15.708 ft---------> 16 ft (in one second)

the answer is
16 ft/sec