Since Bob has a fixed number of pumpkin pies, Catherine must have 55 – 35 = 20 more pumpkin pies than Alice.

Since Catherine has 3 times as many pumpkin pies as Alice, and since Catherine has 20 more pumpkin pies than Alice, you must find a number such that when you add 20 to it, it is 3 times bigger than it was originally. You can find this number using the formula: number = (difference) ÷ (times bigger – 1)

Difference: 20

Times bigger: 3

Number: 20 ÷ (3 –1) = (20 ÷ 2) = 10

Alice has 10 pumpkin pies.

If you don’t know this formula, you can also find the number by trial and error. Start with a number, add 20 to it, and see if it is 3 times bigger.