A cement mixture is composed of 3 elements. By weight, 1/2 of the mixture is sand, 1/5 of the mixture is water, and the remaining 15 pounds of the mixture is gravel. What is the weight of the entire mixture in pounds?  Choose the correct answer: A) 30 B) 40 C) 50 D) 60 E) 80

Rationale: "let the total weight be x . sand content = ( 1 / 2 ) x water content = ( 1 / 5 ) x gravel = x - ( 1 / 2 ) x - ( 1 / 5 ) x = ( 3 / 10 ) x = 15 x = 50 then answer will be c = 50"
Answer: C