Five friends went to a restaurant and agreed to share the bill equally. however, one person forgot his wallet so the other four friends' portions of the bill went up by $4 each. how many dollars was the total bill?

If each of them paid an extra of $4.
4 person paid $4 x 4 = $16 extra

The $16 is the share of the 5th person, which is also each person's share.

$16 x 5 = $80.

Answer: The total bill is $80.