Hi Matthew,

Think about what you would do if you knew the original price and you wanted to calculate the retail price after an 80% markup. Suppose the original price was $\$p.$ To calculate the retail price you would find 80% of $\$p$ and add it to $\$p.$ Thus the retail price would be

\[\$p + 0.80 \times \$p = 1.80 \times \$p.\]

Thus you have

\[1.80 \times \$p = \$99.\]

Since $\$99$ is 1.80 times $\$p$ you know that $\$p$ is $\$99$ divided by 1.80.

I hope this helps,
Penny