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