Multiple choice

X alone can do a work in same time in which Y and Z do it together. If X and Y do it together in 20 days, and Z alone in 100 days, then in how many days can Y alone do it?

  1. 50 days

  2. 60 days

  3. 80 days

  4. None of these

Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Given 1/X = 1/Y + 1/Z and 1/X + 1/Y = 1/20, and Z = 100. Thus 1/Z = 1/100. Substituting: 1/X = 1/Y + 1/100. Then (1/Y + 1/100) + 1/Y = 1/20. So 2/Y = 1/20 - 1/100 = 4/100 = 1/25. Thus Y = 50.