No, I dont think it means only 3 girls paid – it’s still 4 girls paid $2.40 more.
- It is stated in question that 4 girls shared the cost.
- One of them calculated wrongly and divided that amount by 3 (so each share is $2.40 more)
- 4 girls paid that full amount
- Therefore the amount they paid is more than the cost of the cake by ($2.40 * 4 = $9.60)
If you let each correct share = U
4U+9.60 = the full amount paid
This is also equal to (4U/3) *4
So 4U / 3 * 4 = 4U + $9.60
4U = 3U + 7..20
U = $7.20 (the correct amount for each share)