To calculate the percentage of change We:
⇒work out the difference of the two costs
$99-$79=$20
⇒Divide the difference by the original

⇒Multiply by 100 to get the answer as a percentage

× 100
=20.2%
When we have a check:
$99 × 0.202 = 19.998 ≈ $20
║
The right answer