Question

Four taps can individually fill a cistern of water in $1$ hour, $2$ hours, $3$ hours and $6$ hours respectively. If all the four taps are opened simultaneously, the cistern can be filled in how many minutes?

• A. $20$
• B. $30$
• C. $35$
• D. $40$

Answer 1

The correct option is B $30$

$4$ taps individually fill a cistern of water in $1$ hour, $2$ hours, $3$ hours and $6$ hours.

Part of the cistern filled in one hour
$=(1+\frac{1}{2}+\frac{1}{3}+\frac{1}{6})=2$
$\Rightarrow$ Twice the capacity is full in $60$ min.
Therefore, cistern is full in $30$ min.