solving problem..

Minimum total bill equals $27.96 times (100% + 15%)

$27.96 x (115%)

$27.96 x (1.15) = $32.15 total cost which includes the tip.

If you want only the amount of the tip, let's solve that two different ways:

$32.15 - 27.96 = $4.19

0r,

27.96 x 0.15 = $4.19

Doesn't your textbook have example problems, or doesn't your teacher work out examples on the blackboard?

27.96 is nearly 28 $
10% is $2.80
15% is $2.80 + half or $2.80 or $1.40 = $4.20
20% is $2.80 + $2.80 = $5.60
half way between 4.20 and 5.60 is $4.90
Round it up because you are a generous person and give the poor student a $5.00 bill