Prep上的两道数学题，不会，请教

usocean

Pat, Kate, and Mark charged total of 162 hours to a certain project. If Pat charged twice as much time to the project as Kate and 1/3 as much times as Mark, how many more hours did Mark charge to the project than Kate?
A.18; B.36; C.72; D.90; E.108;
我计算的结果是A，但是答案是D

The arithmetic mean and standard deviation of a certain normal distribution are 13.5 and 1.5, respectively. What value is exactly 2 standard deviations less than the mean?
A.10.5; B.11.0; C.11.5; D.12.0; E.12.5;

这个更惨，根本没有读懂题，说的什么意思？

xingchunwang

设pat为p, kate为k，mark为m
则 p=2k，p=1/3m，k=p/2，m=3p， p/2+3p+p=162。p=36 k=18 m=108
108-18=90 答案为正解

xingchunwang

第二题问比13.5 小2个1。5 的值。为10.5

usocean

谢谢。
第一个理解成P=4/3M了，粗心了。
话说第二题也够变态的，把一个13.5-2*1.5=?的问题搞得这么复杂...