Question

Three pipes A, B,C can fill a cistern in $20$ minutes, $15$ minutes and $12$ minutes respectively. The time in minutes that three pipes together will take to fill the cistern, is

• A. $5$ minutes
• B. $10$ minutes
• C. $12$ minutes
• D. $15\frac{2}{3}$ minutes

Answer 1

Answer: (A)

$5$ minutes

Let the capacity of cistern be $60$ units $[L.C.Mof20,15and12]$
A does = $\frac{60}{20}=3units/minute$ B does = $\frac{60}{12}=5units/minute$
(A + B + C)'s per minute work = $(3+4+5)units/minute=12units/minute$
Together they will fill the tank is $\frac{60}{12}=5$ minutes