Each boy paid $3.60 more than his share.

There are 5 boys. So $3.60 x 5 = $18.00

$18.00 was overpaid in total.

Now, 5 boys are supposed to only pay 1/5 of the cost each.

But each boy paid 1/4 of the cost instead

Total fraction of cost they paid
= 1/4 x 5 = 5/4

So 5/4 of the cost was paid,

5/4 - 1 = 1/4

The overpaid amount was 1/4 of the cost

So 1/4 of the cost = $18

Cost = $18 x 4 = $72

Correct amount for each share

= $72 ÷ 5

= $14.40

Units way :


Dividing by 4 means :

We can say the cost is 4 units and each portion/division is 1 unit.

But the boys paid 1 unit each.

Total units they paid

= 1 unit x 5
= 5 units

Total amount they overpaid (in units)

= 5 units - 4 units
= 1 unit

Amount they overpaid ($)
= $3.60 x 5
= $18

So 1 unit = $18

Actual cost = $18 x 4 = $72

Correct share
= $72 ÷ 5
= $14.40

Another fractions way :

Divide by 5 : each boy is supposed to pay only 1/5 of the cost.

Divide by 4 : each boy paid 1/4 of the cost instead.

1/4 - 1/5
= 5/20 - 4/20
= 1/20

Each boy overpaid by 1/20 of the cost.

So 1/20 of the cost = $3.60

Cost = $3.60 x 20 = $72