With a multiple choice question such as this, you may either (1) solve for Agatha's age using the info or (2) determine which of the possible answers (called "distractors") are possible.
Using method (2) [note: often faster], only 30 is a multiple of 5 [see "five times older than"]
Using method (1), let x = Agatha's age today
B = Bob's age today
"Agatha is five times older than her son Bob"
x = 5 * B [let B = Bob's age today]
"Three years ago she was nine times older"
(x-3) = 9 * (B-3) [use (x-3) and (B-3) for three year's ago]
Now, since we don't need B. substitute (x/5) for it: [see first equation]
(x-3) = 9*((x/5)-3)
5x - 15 = 9(x - 15) [multiply all by 5]
5x - 15 = 9x - 135
-4x = -120 [subtract 9x and add 15 to both sides]
x = 30 [divide both sides by (-4)]
Checking is left to you (and it's important).