I got in my car and drove 40 miles to see my cousin at a rate of 20 mph. Then, I left my cousin’s house and drove another 30 miles to the store but this time went 10 mph. What was my average speed for the whole trip?

Answer 1

speed = distance/time, so
(40+30)/(40/20 + 30/10) = (40+30)/(2+3) = 70/5 = 14 mi/hr

Answer 2

d1 = r1*T1
40 = 20T1
T1 = 2h.

d2 = r2*T2
30 = 10T2
T2 = 3h.

d = r*T
40+30 = r*(2+3).
r = 14 mi/h.