Angie is 11, which is 3 years younger than 4 times her sister's age.

[math_celebrity]

Angie is 11, which is 3 years younger than 4 times her sister's age.

Let her sister's age be a. We're given the following equation:
4a - 3 = 11

To solve for a, we type this equation into our math engine and we get:
a = 3.5