first note that 35 mins = 7/12 hours. we need our units to be consistent.

Let be the time it takes to drive 150 miles at a speed of . then we know

.................(1)

if we increase the speed by 10, the time falls by 7/12, so that

................(2)

thus we have two equations, two unknowns. we can solve simultaneously. equate the 's to get

multiply through by , we get:

can you continue?