2 PS

1. A certain shade of gray paint is obtained by mixing 3 parts of white paint with 5 parts of black paint. If 2 gallons of the mixture is needed and the individual coors can be purchased only in one-gallon or half-gallon cans, what is the least amount of paint in gallons that must be purchased in order to measure out the portions needed for the mixture?
B 2.5
E 4

white paint: 1.5 gallon, black paint: 2.5 gallon, total : 4 gallon
but the key is B. How to get it?

2. Of the science books in a certain supply room, 50 are on botany, 65 are on zoology, 90 are on physics, 50 are on geology, and 110 are on chemistry. If science books are removed randomly from the supply room, how many must be removed to ensure that 80 of the books removed are on the same science?
A 81
B 159
C 166
D 285
E 324

我是用排除法得到 E。有其他方法吗？