How do you know the shaded circle are 3/4 of the entire circle on page 271 question 4

Angles *A* and *C *are 90° angles (the question tells you *ABCD* is a square) and we know *A* and *C* are the centers of their respective circles (the question tells you that, too).

90° is 1/4 of 360°, so each circle is missing 1/4 of itself.