Let x = the amount of gas pumped to the smaller tank
y = the amount of gas pumped to the larger tank
Since the total gas pumped to the two tanks is 25 gallons of gas, we can say that:
x + y = 25
We just have to solve x in terms of y to express the amount of gas pumped into the smaller tank in terms of the amount pumped into the larger tank.
We can get:
x = 25 - y