Question

Use a ratio box to solve this problem. The cost of a 10-minute call to Boise decreased by 20%. If the cost before the decrease was $3.40, what was the cost after the decrease?

Answer 1

The cost after the 20% decrease is $2.72

Explanation:

Here, we are trying to calculate the cost of a 10-minute call before there was a decrease in the amount charged for the call.

We proceed mathematically as follows,

To calculate percentage decrease, we can employ the formula;

(old value- new value)/old value * 100%

Here our old value or cost was $3.40 and the percentage decrease is 20%

we plug these values;

Let the new cost it value be x

(3.4-x)/3.4 * 100 = 20

100(3.4-x) = 20 * 3.4

340 - 100x = 68

100x = 340 -68

100x = 272

x = 272/100

x = $2.72

Answer 2

$2.72

Explanation:

If the price decreased by 20%, the final price will be 100% - 20% = 80% of the inicial price.

So, we can write the ratio box:

Old value New value

Percent value: 100% 80%

Value: $3.40 X

Then we can solve with a rule of three:

100% -> $3.40

80% -> X

X = 0.8 * 3.4 = $2.72