Tap A and B can fill a cistern together in 2.4 hours. The water from Tap A flows at the rate of 100 litre per hour to fill the cistern while Tap B has the capacity to fill the entire cistern in just 4 hours. Find how much water can the cistern hold?

Let Tap A fill the cistern completely in A hours.
So in 1 hour, it fills 1/A amount of the cistern

Also in 1 hour in Tap B fills in 1/4 amount of the cistern
Together they fill the cistern in 2.4 hours
So, Also in 1-hour together they fill in (1/2.4)amount of the cistern
So, Also in 1-hour cistern filled by both is given by 1/A + 1/4 = 1/2.4
∴ 1/A = 1/2.4 - 1/4 = 1/6

∴ Pipe A can fill 1/6^th tank in 1 hour
∴ Pipe a fills the tank completely in 6 hours.
It has a rate of 100-litre water per hour,
So, in 6 hours it gives out 6 x 100 = 600 litres
In 6 hours cistern is full, so capacity = 600 litres.