At the start of the week, you could run for 8 minutes. by the end of the week, you could run for 15 minutes. which proportion could you use to find the percent increase in your running time?
7/15=n/100
8/7=n/100
7/8=n/100
8/15=n/100

The formula for finding percent increase is:
(Final - Initial)/Final * 100 = n
Divide both sides by 100 and you get:
(final-initial)/Final = n/100
Plug in all the numbers, and you get:
(15-8)/15 = n/100, or
7/15 = n/100

Hope this helped!! :D