apples - 2

pears - 3

prunes - 4

You want to max out prunes, so this means that Jeremy would buy as many prunes as he could until he did not have enough money left to buy any more prunes.

So there are a number of ways to approach this.

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Subtraction Method- subtract 4 from the total for every box you want him to buy until you cannot subtract any more:

1 box: 23 - 4 = 19

2 boxes: 19 - 4 = 15

3 boxes: 15 - 4 = 11

4 boxes: 11 - 4 = 7

5 boxes: 7 - 4 = 3

And since he only has 3 leftover after buying 5 boxes, he cannot afford any more prunes. The remaining 3 could be spent buying pears.

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Addition Method- add 4 for every box he buys until he cannot buy any more without going over the total money spent:

1 box: 0 + 4 = 4

2 boxes: 4+4 = 8

3 boxes: 8 + 4 = 12

4 boxes: 12 + 4 = 16

5 boxes: 16 + 4 = 20

And since 20 + 4 = 24 which is more than the 23 he spent, he cannot buy any more.

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Multiplication Method- know your multiplication tables, and see what is the greatest coefficient of 4 that leaves the total under 23:

let x be the number of prunes he bought

4x < 23

you can see that 4*5=20 and 4*6 = 24. since 20 < 23 and 24 > 23, you know that he must have spent 20, so x = 5, so he bought 5 boxes.

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Division Method- divide the money spent buy the price per item, discard any leftover since he cannot buy a partial box:

4/23 = 5 remainder 3

I personally used multiplication, to solve this one, I think it is the best option by far, if you become familiar enough with it, you should be able to do these in your head.

Here is another to try out the technique on: If Jeremy spent 35 Euros, what is the most prunes he could have bought? What is the most pears he could have bought? What is the most apples he could have bought?