The case of the missing 10 cents

Suppose Ali, Bala, and John buy a ruler that costs $2.50. They each have $1, and they pool a total of $3 to purchase the latter. So the change is 50 cents. Now, if we give each boy 10 cents back (since we cannot split 50 cents evenly among the 3 boys), we would have 20 cents left. But by doing so, each boy would have contributed 90 cents. In all, they contribute 90 X 3 = $2.70. If I add back the leftover 20 cents, we would have

$2.70 + $0.20=$2.90.

So where did the 10 cents go?