10. Mary's travel time and speed vary INVERSELY. Mary drives home from work in 40 minutes driving 55 mph. If Mary needs to get home in 35 minutes, how fast does she need to drive? (Rounded to the nearest whole number!)

Answer 1

time = constant / speed
t = k/s
40 = k / 55
k = 2200
so
t = 2200 / s
35 = 2200 / s
s = 2200/35 = 62.86 or 63 mph

Answer 2

distance = rate x time
d = 55 mph x (40/60) = 36.6 miles to her house.
Then 36.6 = rate x (35/60) so rate = 36.6 x 60/35 = ? mph