yeah, that's the +/- problem
<<270 + 20(what the fourth guy kept) = 290.>> WRONG. 270
- 20 (for the 4th guy. If you have to pay something you can't add it to your money, you have to subtract it)
3x0,90= 2,70
Bread= 2,50
Money left: 0,20
4th guy took 0,20
Money: 0,00
The mistake is, that you calculate with an entire price of 3,00, but that's wrong! The 3 guys only pay for bread (2,50) and the 0,20 for their friend. So it's 2,70 (3x0,90).
If you want to solve it with the 3,00:
4th guy has 3,00
pays 2,50 for bread
money left: 0,50
4th guy takes 0,20
money left: 0,30
each of the 3 guys get 0,10
Money left: 0,00