Let g = number of gallons after m minutes.
At time 0, g = 35.
At time zero, the tank has the full 35 gallons and has not lost any water yet.
After 1 minute, the number of gallons goes down by 1/4 = 0.25, so at m = 1,
g = 35 - 0.25
After 2 minutes, the tanks has lost two 1/4-gallon amounts, or 0.25 * 2, so at the end of the second minute, when m = 2, the volume is
g = 35 - 0.25 * 2
You can see that for each minute that passes, the volume goes down by an other 0.25 gallons, so at the end of the third minute, when m = 3,
g = 35 - 0.25 * 3
Now we can generalize. At the end of minute m, the tank has lost 0.25 * m gallons of water, so at minute m, the tank has
g = 35 - 0.25m
That is the expression.
