At the end of the month, a certain ocean desalination plant’s reservoir contained 30 million gallons of water. This amount is twice the normal level. If this amount represents 75% of the reservoir’s total capacity, how many million gallons short of total capacity is the normal level?  Pick one:
A) 10
B) 15
C) 20
D) 25
E) 30

Rationale: "the q talks of total capacity , normal level , present level , shortage etc . . so it is all about not going wrong in these terms 30 mg = 75 % of total . . total = 30 / . 75 = 40 mg . . normal level = 1 / 2 of 40 = 20 mg . . shortage of normal level = 40 - 20 = 20 mg . . c"
Answer: C