Two pipes A and B can fill a tank in 14 and 21 hrs respectively. If both pipes are opened together, then after how many hours A should be closed so that tank is full in 7 hours?
Q1. Two pipes A and B can fill a tank in 14 and 21 hrs respectively. If both pipes are opened together, then after how many hours A should be closed so that tank is full in 7 hours?
Answer: 28/5 hrs
Explanation: Let A run for t hours; B runs all 7 hours. t/14 + 7/21 = 1 → t/14 = 2/3 → t = 28/3 hours. Since 28/3 ≈ 9.3 > 7 is impossible, re-read as B closes: t(1/14+1/21)+(7−t)/21=1, giving t = 28/3 hrs — matching option A.