They have emptied 1/2 to an oil tank; then they emptied another half of what was left; then the rest of 11/15 emptied. Now, there are 26 liters in

the tank. How many liters were at first?

add up the fractions and subtract from 1

That is how much was left. Equate that to 26.

For example if, 2/13 was left, then

2/13 x = 26
x = 169