Since both Zoe and Mabel gave half of what they baked to each other, they both had equal number of chocolate cookies and equal number of butter cookies.

(a) Even after Mabel ate 3 chocolate cookies, her remaining number of chocolate cookies was 3 times more than that of butter cookies.

There were more chocolate cookies than butter cookies at first.

C : (B – 5) = 4 : 1

C = 4(B – 5) = 4B – 20

(C – 3) : B = 3 : 1

C – 3 = 3B

(4B – 20) – 3 = 3B

B = 23

C = 4B – 20 = (4 × 23) – 20 = 72

(b) Number of chocolate cookies Zoe baked -> 2C = 144

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