Hi Jessica,

Following pls find my worked solution:

*** - alignment purpose

Apply <work backwards> method, we have ...

<After>

A : B : C

2 : 5 : 9

<process>

C: 3x original number

B: +70

A: (3/5) given away

<Before>

C: 3u *** {9u ÷ 3}

B: 5u - 70

A: 5u *** {since (2/5) = 2u}

Given that total number of cards in Box A, Box B and Box C was 1022 at first, we have ...

13u --> 1022 + 70 = 1092

1u --> 84

(a)

C - A (end) = 9u - 2u = 7u

7u --> 7 × 84 = **588**

(b)

% increase in number of cards in box B:

Number of cards in B (before) = 5u - 70

5u - 70 --> (5 × 84) - 70 = 420 - 70 = 350

(70 / 350) × 100 = **20%**

========

Trust this helps.

Do let me know again if there's further clarification.

Cheers,

Edward