you can solve this with proportion - first way
1.25 $/gal - 100%
1.75 $/gal - x%
1.25 : 1.75 = 100 : x => 1.25x = 1.75 * 100 => 1.25x = 175 => x= 175/1.25 =>
x= 140% => Increase is 40%
The second way, which is easier
1.25 *x = 1.75 => x= 1.75/1.25 = 1.4
Integer 1 mean 100% and 0.4 mean 40%
We can conclude that increase is 40%
Good luck!!!