Question 692583
 David is 1/4 mile from home bicycling at 21 mph, when his brother takes off on his bike from home trying to catch him.
 How fast must David’s brother go to catch David a mile from home?
:
Brother has to ride 1 mile while David rides 3/4 of a mile (use .75 mi)
:
Let s = brothers required speed, to accomplish this
:
Write a time equation, time = dist/speed 
:
{{{.75/21}}} = {{{1/s}}}
cross multiply
.75s = 1 * 21
s = {{{21/.75}}}
s = 28 mph
;
:
Find the times, should be equal
1/28 = .0357 hrs
.75/21 = .0357