A tank can hold 50 liters of water. At present, it is only 30% full. How many liters of water shall I put in the tank, so that is it 50% full?

Calculate how many liters you need to be half full:

50 liters x 50% = 50 x 0.5 = 25 liters to be half filled.

Calculate how many liters are in the tank now:

50 x 0.30 = 15 liters

Now subtract to find how many more liters are needed:

25 - 15 = 10 more liters.