96 Part 1: The World of Numbers
To find 15 or 20 percent quickly by mental math, remember that 10 percent of a number is one
tenth of the number, or the number divided by ten. Dividing by ten is just moving the decimal
point one place left. 10 percent of $27.46 is $2.75 (rounded up from $2.746). Double that to find
20 percent, and the tip is $5.50, or if you want 15 percent, take half of $2.75, which is about $1.38,
and add that to the $2.75, to get $4.13.CHECK POINT- Find the simple interest on $18,000 invested at a rate of 4% for 5 years.
- If you paid $130 simple interest on a loan of $1,000 for 2 years, what was the
interest rate? - How much tax will you pay on a purchase of $175 if the sales tax rate in your
area is 4.7%? - If the dinner bill for your family is $35.84, and you want to leave a 20% tip for
your server, what is the amount of the tip you should leave? - A restaurant that sells a fixed price dinner for $22 per person tells you that
for groups of 8 or more, they will automatically add an 18% tip to the bill. If
you and seven friends plan to go for dinner, how much should each person be
prepared to pay for the dinner and the tip?
Percent Increase and Percent Decrease
You’ll sometimes see an advertisement that claims there’s been a 40 percent increase in the num-
ber of vitamins in a bottle, or that there’s been a 30 percent decrease in the price of something.
These are examples of percent increase or percent decrease. They compare the change, whether
increase or decrease, to the original amount. And it’s amazing how many times those ads have the
calculations wrong.
There’s no reason for you to get them wrong. The original amount is the whole, and the change
is the part. To calculate a percent increase or a percent decrease:- Identify the original amount. That’s the whole.
- Calculate increase or decrease. That’s the part.
- Use
part
whole%
100
to calculate the percentage.