Question

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?

Answer 1

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.

Answer 2

Hey there :)

1 ) Five friends went to a restaurant with an agreement to share the bill
2 ) One forgets so four friends part of bill rose by $4 each

Let p represent the total amount of bill

Translating (1) into mathematics:
$\frac{p}{5}$      ( Original agreement )

Translating (2) into mathematics:
$\frac{p}{4}$      ( Share among four )

Portions of bill increases by
4

Therefore, 
$\frac{p}{4} - \frac{p}{5} = 4$

Change to same denominators 
$\frac{p(5)}{4(5)}- \frac{p(4)}{5(4)} =4$
$\frac{5p}{20} \frac{4p}{20} = 4$
p = $80

The total bill was $80