When Cheenu was a boy he could run $15$ miles in $3$ hours and $30$ minutes. As an old man he can now walk $10$ miles in $4$ hours. How many minutes longer does it take for him to travel a mile now compared to when he was a boy?

A. $6$

B. $10$

C. $15$

D. $18$

E. $30$

1. 2016 AMC 8 官方解析

翻译：切努还是个孩子的时候，跑15英里用时 3时 30分，现在成年了，步行10英里用4小时，
那么他现在步行1英里用时比孩童是跑1英里多用（　）分钟？

步行1英里用时$4 \times 60 \div 10 = 24$（分）；

跑1英里用时$(3 \times 60 + 30) \div 15 = 14$（分）；

多用时：$24 - 14 = 10$（分）．

When Cheenu was a boy, he could run 15 miles in 3 hours and 30 minutes $= 3 \times 60 + 30$ minutes $= 210$ minutes, thus running $\frac{210}{15} = 14$ minutes per mile. When he is an old man, he can walk 10 miles in 4 hours $= 4 \times 60$ minutes $= 240$ minutes, thus walking $\frac{240}{10} = 24$ minutes per mile. Therefore it takes him B 10 minutes longer to walk a mile now compared to when he was a boy.