40% is 2/5. 30% is 3/10

2/5 in Box A = 3/10 in Box B

We can think of 2/5 as 2 units out of 5 units and 3 units out of 10 units.

Since the absolute number of marbles is the same for 2/5 of Box A and 3/10 of Box B, the number of units should be the same. Make the numerator the same by changing the fraction.

common multiple of 2 and 3 is 6.

2/5 = 6/15
3/10 = 6/20


So we can say there are 15 units of marbles in Box A and 20 units of marbles in Box B, a total of 35 units.


Now Box C has 80% of Box A and B's total marbles.

80% is 4/5.

So Box C has 4/5 x 35 units = 28 units of marbles.

25% were removed from C.
25% is ¼.
So ¼ x 28 units = 7 units of marbles were removed from Box C.

28 - 7 = 21

21 units of marbles were left.

Difference between marbles in Box C and A now

= 21 units - 15 units
= 6 units

So Box C has 6 units more marbles than Box A.

6 units = 78
1 unit = 78 ÷ 6 = 13 marbles.


Number of marbles in Box B

= 20 units
= 20 x 13
= 260

Thank you

Stiill editing. Hold on.

Done editing. Welcome and do clarify your doubts about the working if you have any.

Thank you