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?

The cost after the 20% decrease is $2.72

Step-by-step 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