To check right
Why you divide by 5?
Why you also multiply by 7.

Sorry I am trying to find a way to explain to an 9 year old.

Have to split the model in a way that the units just nice that when K give to B, both will have equal number of stickers.

If never split the model into 14 smaller units, there is no way K can give to B such that the number of stickers both of them in the end is equal.

If never split, K will have 6 units instead and B 1 unit, which means K has 5 units more than B, and 5 divided by 2 will be 2.5 units which the model cannot show nicely that both of them will have equal number of stickers in the end.

That's why have to split the model into K (6 times 2 = 12 units and B is 1 times 2 = 2 units)

Multiply by 7 is because whether who give who, the answer total among K and B will always be the same.

So multiply by 7 here is based on "at first scenario" . K at first has 6 units (also means 12 units because of the splitting) , which includes the 300 that haven't give B. And B at first only has 1 unit (also means 2 units due to splitting), so total 12 plus 2 units is 14 units, and 1 smaller unit is 60, therefore 14 times 60.