Tank $A$ contains $5$ times as much water as Tank $B$. How much water must be transfer from Tank $A$ to Tank $B$ so that each tank contains $45$ liters of water?

The correct option is A $30litres$
Let quantity of water in tank $B$ be $x$ litres.
Then quantity of water in the tank $A=5x$ litres
Total water in both the tanks $=x+5x$
$=90liters$
$\Rightarrow 6x=90\Rightarrow x=15$
$\therefore$ Water in tank $B=15litres$
$\Rightarrow$ Water in tank $A=15\times 5=75litres$
So, quantity of water to be transferred from tank $A$ to tank $B$, so that each tank contains $45litres=75-30=45litres$
Therefore, $30liters$ of water should be transferred.