Question 21420
This is a rate problem. The key is: (rate * time) = distance.
The rate for Sara is ( 1 lap / 2.5 min )
She is keeping up this rate for 13 minutes.
( 1 lap / 2.5 min ) * 13 min = L lap

Solve this for L.

Think of it this way.
If she can lap the pool in 2.5 minutes, how much of the pool can she swim in just one minute? If she lapped the pool in 2 minutes, then in 1 minute, she'd 
swim 1/2 the length of the pool. 1/2.5 will give you the amount she can
swim in just 1 minute.
The answer is .4, so that is the fraction of the pool she can swim in 1 minute.
In 13 minutes, she swims 13 times that or 13 X .4 = 5.2 laps. Hope this helps