Lesson 9 — Activity 2:

Consumers' Corner

Using Formulas to Find Missing Information

Welcome to the Mathematics Store, where we will present you with many wonderful options!

Choose among a large variety of smartphones, calling plans, and satellite packages. Enjoy!

Smartphones come with numerous options, such as cameras, text messaging, Internet browsing, data minutes, and other features. Some of these features are useful; others are just nice extras. But all of them add to the cost of the phone, even if the advertisements claim that the extras are free. When more technology is added, the consumer pays for all of the extras.

When you purchase a smartphone, you must consider carefully what you can afford. Remember also that some features such as sending pictures and text messages cost more every time you use them.

It's impossible to list every variety of smartphone plan, but below are some examples.

Which of the three plans is the best buy on a cost per minute basis? You can use a formula to find the missing information! The formula would be: Cost (C) divided by time (T) gives cost per minute (M). *Round to the nearest cent.

Plan A: $35 ÷ 200 min. = $0.18 per min. = 18¢/min. Plan B: $45 ÷ 300 min. = $0.15 per min. = 15¢/min. Plan C: $50 ÷ 400 min. = $0.13 per min. = 13¢/min. Therefore, the most economical plan is Plan C: 400 minutes for $50 because each minute of talk costs only 13¢ per minute!

You can also use formulas to find out other information, such as how many minutes you used or how much data you used in a month based on the cost of your bill.

For example, say you received your monthly smartphone bill for $70 and you want to find out how many minutes you used. You could use the following formula:

0.30M + 25 = C

(Where M = Minutes used that month x $0.30 per minute + $25, which is the base cost of your plan.)

You know that C = $70 (which is the total cost of your bill), therefore:

0.30M + 25 = 70

So now you need to find out how many minutes (M) x 0.30 (which is 30¢ per minute) + 25 (which is the base cost of your plan) is equal to 70!

To do this, you would do the following:

70 – 25 = 45 (or, the total cost of your bill minus the base cost of your plan, which equals how much you spent on your minutes). Now all you need to do is find out how many minutes you used by dividing: 45 ÷ 0.30 = 150. Therefore, you used 150 minutes this month!

  Self-check!