Math IS Real Life: August 2015 – Discounts!

It’s the first Wednesday of August which means it’s time for our monthly linky – Math IS Real Life!! If you want to see how the linky works, or just want other real world math ideas, check out our Pinterest Board of all the posts so that you can look back and find some great ideas and REAL pictures to use in your classroom!

If you are linking up, please include the below picture link back to this blog post

mathisreallife-revised

 A monthly REAL WORLD math blog link-up hosted by MissMathDork,

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My MIL and I are really big into crafting and DIY projects.  Anyone who has an interest in either knows how expensive that can get!  Soooo….  needless to say we’re always looking for a deal.  I remember one of the many times my MIL and I went into a store together.  We were in a craft store and it was a few days after Christmas.  She had just found some adorable Christmas ornaments on mega discount.  They were originally something rather crazy like $13 a piece and were discounted 80% off.  I said something to the effect of “wow, $2.60 a piece that’s not too bad.”  She just kind of looked at me and asked how I calculated that so quickly.  I taught her how to do a mental discount in you head (this is NOT how I originally teach it to kiddos, but it fun to see if you can get them to spot the ‘speedy way’).

$13.00

1) make sure both decimal places are showing in the number (if not add .00 to the end)

2) truncate the tenths digit (13.0)

3) slide decimal to the left (1.30)

VOILA that’s 10 percent of the original number.

Most people would then multiply by 8 and then subtract from the original price, but I do something a bit easier.  I think about 80% in relation to 100%. If the price is 80% off, then I am paying 20% MUCH easier to calculate.

1.30 x 2 = $2.60.

This is such an easy mental math “lesson” to teach your kids and use when you are in the stores.  It also helps with calculating tips in restaurants!

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Sometimes the discount is VERY simple to figure out.  50%, just cut the number in half.

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70% off? Just find 10% and multiply that number by 3.
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60%, no worries, find 10% and multiply by 4.  The more the kiddos play with the numbers more they start making relationships within sums of 10 and 100.  HUGE place value number sense!

A great place to practice these skills is in a store that has “cheat sheet” discounts.  Ask a cashier if they have left over sale signs. Fold the sale sign in half so that the Now/Regular part is hiding and then have students calculate various “original” prices.  Great for a math station in your classroom!

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  Don’t forget to check out the other MIRL posts below!  Check back over the next few days – more will be added!!


Categories: #mirl, computation and estimation, DIY, math is real life, mathematical reasoning, and mental math.

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