In general, people tend to think of percent increases and decreases as absolute measurements. They should be thought of as relative measurements
. This post includes sample problems that cover global warming, subway fare hikes, the stock market, and the unreasonably high cost of coffee. New challenges will be sent out on a weekly basis.
Why Percents are tricky, and should not be thought of as absolute measurements
Consider this…
On Monday, the price of pair of gloves was
$10
On Tuesday, the price increased by 20% – bringing the price up to
$12
On Wednesday, the price decreased by 20% – what was the final price?
Most people answer:
$10
Correct answer:
$9.60
Decomposing % problems into starting points and ending points – kind of like a journey
Let’s unpack the first example point by point:
On Monday
Our starting point =
$10
Nothing happens on Monday
Our ending point =
$10
On Tuesday
Our starting point =
$10
Something happens – the price increases by 20%
20% of what? 20% of our starting point… which is $10
20% of $10 = $2
If the price increases by $2, the price of gloves is now $12
Our ending point =
$12
On Wednesday
Our starting point =
$12
Something happens – the price decreases by 20%
20% of what? 20% of our starting point… which is $12
20% of $12 = $2.40
If the price decreases by $2.40, the price of gloves is now $9.60
Our ending point =
$9.60
Typical SHSAT examples
Shares of XYZ stock were trading at $130 this morning, but later fell by 10%. The ending share price was $117.
The tree in our back yard was 60 inches tall. A year later, it grew 5%. The new height was 63 inches.
The price of a blouse is $50 without tax. With tax, it’s $54.50. The 9% sales tax added $4.50 to the cost.
A subway token used to be $1.25. It went up 60%, making the new cost $2.00.
A car’s fuel efficiency used to be 20mpg. Major engineering changes raised this to 30mpg – a 50% improvement.
Don’t forget! % OF
some number is just multiplication by that number
Any percentage OF a number is the decimal equivalent of the percentage multiplied by that number. In math speak, when you see/hear the word OF… it’s an indication that you have to multiply.
20% of 80 = .2 x 80 =
16
120% of 100 = 1.2 x 100 =
120
0% of 1,000,000 = 0 x 1000000 =
0
The other area you see “OF” expressions implying multiplication is fractions
. For example…
1/5 of 80 = 1/5 x 80 =
16
6/5 of 100 = 6/5 x 100 =
120
0/2 of 1,000,000 = 0/2 x 1000000 = 0
Real-world example:
97% of scientists agree that humans are the primary cause of global warming
There are roughly 7 million scientists employed in the United States
If we applied this percentage (97%) to the roughly 7 million scientists employed in the United States, we would get 6,790,000 scientists
This exceeds the population of Indiana
Weekly Challenge: 5/16/2021 (Percents)
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This week’s theme: 🇵🇷 The Bronx
A shout-out to my Puerto Rican cousins – the Vegas.