Example: Bought a watch for $15 and sold it for $18; what per cent did I make?
| .15 | ) | 3.00 |
| 20 | ||
| Ans. 20% | ||
Here, $15.00 is the base, and ($18 - $15) $3.00, the gain or percentage. Now, as 1% of 15.00 is .15, it is evident that 3.00 is as many per cent of 15.00, as .15 is contained times is 3.00, which is 20.
Proof: 20% or 1⁄5 of $15 = $3.
The percentage and rate given, to find the base.
Rule.—Divide the percentage by the rate per cent expressed decimally.
Example: Received $6.40, percentage or interest, for money loaned at 4%, what was the base or principal?
| .04 | ) | 6.40 |
| Ans. $160 | ||
If $1 produces .04 (4 cents) in a certain time, $6.40 must be the percentage of as many dollars as .04 is contained times in $6.40, which is 160.
Proof: 4% of $160 (160 × .04) = $6.40.