How to teach your kid to work out sale prices and find the best deal

"20% off!" "Buy the big box and save!" Stores are full of math, and a lot of it is designed to make you spend more, not less. A kid who can work out a sale price or compare two sizes of cereal is doing real sixth-grade math, and protecting the family budget at the same time.

Why it's worth doing in a store

In schools using the Common Core math standards, sixth graders learn to find a percent of a quantity, with 30% meaning 30/100 of it, and to solve unit rate problems, including unit pricing. These are some of the most-used math skills in adult life, and the grocery store is a free lab.

The Consumer Financial Protection Bureau's Money as You Grow program calls it a good habit to shop around and compare prices before you buy, and suggests showing kids how much money is saved with coupons and discounts.

Keep it fun, not tense. A University of Chicago study found that when parents who felt anxious about math helped with homework often, their children learned less math and grew more anxious about it. Store math works best as a game: "Can you beat the calculator?" "Can you find a better deal than me?"

Skill 1: Work out a 20%-off price

The quest uses a trick that works for any percent that's a multiple of 10:

1. Read the original price. Say, $24.00.
2. Find 10%: move the decimal one place left. $24.00 → $2.40.
3. Double it to get 20%. $2.40 × 2 = $4.80.
4. Take that off the price. $24.00 − $4.80 = $19.20.

Same trick, other percents:

Then ask the real question: is $19.20 a good deal? Do we need it? A sale on something you don't need isn't savings.

Skill 2: Compare unit prices

Bigger isn't always cheaper per ounce. The unit price tells you the truth.

1. Find the price and size of each item.
2. Divide each price by its size.
3. Compare the two unit prices.
4. Say which is the better value.

An example:

The big box costs more per ounce. Surprises like that are what make this skill stick.

Shelf tags often show the unit price already, in small print. Have your child work it out first, then check the tag. Watch for tags that use different units for different sizes, like per ounce on one and per pound on another. That's a good catch for a sharp-eyed kid. (More on reading unit prices in shopping smart.)

The challenge: best value on five items

The mastery goal is to find the best value on five items in one shopping trip.

1. Take a list of five items.
2. Find at least two choices for each: different sizes, brands or store brands.
3. Work out the unit price of each choice.
4. Pick the best value for all five.

Bring paper and a pencil, or let your child use a calculator. The thinking matters more than doing it in their head.

Afterward: add up how much you saved compared to picking the first choice each time. Kids are often amazed.

Value isn't only price. The cheapest peanut butter isn't a deal if nobody will eat it. The biggest bag of spinach isn't a deal if half goes bad. Talk about it: "When is the bigger size not the better value?"

Coaching tips

• Let them be the family expert. Ask them, "Which one should we get?"
• Round first, then calculate. $2.97 is about $3. Estimating catches calculator mistakes.
• Celebrate catches. If they find a sneaky deal, or a fake one, make a big deal of it.

Make it a quest

In Family Quests, this is the Deal Detective skill. Practice quests are work out a 20% off price at the store and compare unit prices on two items, and the mastery quest is find the best value on five items in one shopping trip. Start free.

Next: teach your kid to track data and make charts, or read about shopping smart.