Units & Parts Concept (Proportional Change)

Set 1 (Before) vs Set 2 (Proportional Change)
Amount (After)

First
Mrs Kumar bought 2/3 as many chocolate muffins as blueberry muffins.
After she packed 3 chocolate muffins and 5 blueberry muffins into each box, she had 13 chocolate and 11 blueberry muffins left.
How many chocolate muffins did Mrs Kumar buy?

(Before)
Chocolate : Blueberry
= 2u : 3u

(Change)
Chocolate: Blueberry
= 3p : 5p

(Chocolate)
2u - 3p = 13
6u - 9p = 39

(Blueberry)
3u - 5p = 11
6u - 10p = 22

-9p - (-10p) = 39 - 22
1p = 17

Chocolate (Before)
= 2u
= 3p + 13
= 3 (17) + 13 = 64

Set 1 (Before), Set 2 (Proportional Change)
Amount (2 changes)

First
A box contains red marbles and white marbles. 80% of the marbles in the box are red marbles. 28 red marbles and 2 white marbles are removed from the box and the remaining marbles are put into groups of 9 marbles. In each group of 9 marbles, there are 7 red marbles. Find the number of marbles in the box at first.

Second
Mr Tan baked some mooncakes. There were 25% fewer pandan mooncakes than lotus paste mooncakes. He took 20 pandan mooncakes to give to his friends and baked another 13 lotus paste mooncakes. He packed the rest into boxes of 7 mooncakes each. If there were 5 lotus paste mooncakes in each box, how many pandan mooncakes did he bake at first?

Third
In the morning, Smith baked 2/3 as many vegetable pies as curry pies. At noon, he baked 18 vegetable pies and 15 curry pies. After that Smith packed the pies into boxes of 9. In each box, 4 of the pies were vegetable pies. Find the number of vegetable pies Smith baked in the morning.

Fourth
Gareth had 40% as many red shirts as blue shirts at first. He bought  another 80 red shirts and said away 70 blue shirts. He packed the remaining shirts into packs of 11. In each pack. there were 7 red shirt and 4 blue shirts. How many shirts did Gareth have at first?