Problem: An automobile went up a hill at a speed of 10 miles an hour and down the same distance at a speed of 20 miles an hour. The average speed for the round trip was:
Answer Choices:
A. 1221 mph
B. 1331 mph
C. 1421 mph
D. 15 mph
E. (none of these)
Solution:
t1=10d,t2=20d,t1+t2=d(101+201);
∴A=d[(1/10)+(1/20)]2d=340=1321(mph);
or
A1=21(101+201);∴A=340