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​