Hello, Trappl!

You're misreading the percents.

Also, is there some information missing?

As stated, the problem has no solution.

There is water tank with 1,000,000 litres of water and has been tainted with arsenic.

The % of arsenic is 0.12%. .This is 0.0012

To make it safe, it needs to diluted to a level of 0.001%. .This is 0.00001

The tank hold 45 000 000 litres of water

Water can be pumped in at 40,000 litres per hour.

Presently the tank has liters of arsenic.

We will add liters of water.

Then: .

. . . . .

Hence, we must add 119 million liters of water.

But the tank hold only 45 million liters.

We're stuck . . .