EXAMPLE: I bought flour at $3.50 per barrel. For what must I sell it to
gain 20%?
SOLUTION: I must sell it for 100% of the cost plus 20% of the cost, or
120% of the cost.
120% of $3.50 = $4.20.
∴ I must sell it at $4.20.
EXAMPLE: I sold my carriage for 80% of its cost and received $90 for it.
What was the cost?