Question 1077979
Tommy has a head start of 5 x 2, or 10 miles.Then Zach runs at 8 mph. Let x be the amount of time Zach runs. Then:
(10+5x)²+(8x)²=16²
100+100x+25x²+64x²=256
89x²+100x-156=0
(89x-78)(x+2)=0
x=-2 or 78/89 
We get a positive value of 78/89, or 0.8764 hours before they are 16 miles apart. ☺☺☺☺