Pipe A can fill a tank of capacity 350 litres in $3\frac{1}{2}$ minutes. Pipe B can fill a tank of capacity 780 litres in $8\frac{2}{3}$ minutes. How long (in min)will it take to fill a tank of capacity 1615 litres, if both pipes are opened together?

$8\frac{1}{2}$

A = 350 L = $3\frac{1}{2}$ min = \(\frac{7}{2}\)

B = 780 L = $8\frac{2}{3}$ min = \(\frac{26}{3}\)

Therefore, Efficiency of A and B,

⇒ A =\(\frac{350}{7/2}\), B = \(\frac{780}{26/3}\)

⇒ A = 100, B = 90,  

⇒ Time taken by A + B to fill 1615 L of tank = \(\frac{1615}{100+90}\) = \(\frac{1615}{190}\) = $8\frac{85}{190}$ = $8\frac{1}{2}$ minutes.