Mentally Calculating %

We revisited mental strategies for calculating % to enquire into our central idea:

We use fractions, decimals and percentages in a variety of ways in real life situations.


To tune back in, we did a think-pair-share about different ways we use % in real life situations.

Some of our ideas:

We wondered about how much people tip waiters in the U.S and so found out it was 15-20%.  We discussed how Americans really need to learn how to mentally calculate % a lot!

We then revisited mental strategies we had enquired into for shop discount sales and had a minute to practise these strategies mentally.

We started with 10% discounts.  Kids know they can either choose questions in the easier or more challenging columns. We also talked about how we are building our neuron pathways by practising these strategies.

We repeated this for 20%, 25% and 75% discounts and discussed mental strategies we found useful and sometimes unuseful according to the number. 

We found it interesting that calculating 75% off a product is actually easier than 25% since we don't need to subtract from the original price!

Our strategies often led us to using fractions so we found those connections interesting in our discussions too.    We found 40% a bit tricky to calculate mentally.  We came up with several strategies but couldn't pinpoint and which would be the easiest: 

Someone remarked that if it's a 40% discount sign, we should just hand over our card and round it roughly to 50%.  We liked that idea a lot. 

After this, groups had a few minutes to create a list of activities they could make that would help us further build our neuron pathways by practising to mentally calculate % discounts.

We then chose our best idea from the list and started making them.

Some really creative ideas emerged!!

This group created a school supply shop with products marked down at different % for others to solve the new prices of:

If you mentally calculated the new price of an item you could 'buy' it.  Later, valued customers were given vouchers to receive an extra 25% off the discounted prices!  This meant the children needed to mentally calculate even more!

Another group created a fast-food restaurant.  The specials were marked down by % for the others to mentally calculate and we needed to mentally calculate the tips needed to be given from the bill:

Another group decided that ALL the products in their shop would be reduced by 65% for others to solve mentally:

This group created a game where the participants pulled out a number and then needed to mentally calculate 10% off. 

This group's restaurant had an interesting twist.  Each person picked up a % card.  That card dictated how much they had to mentally calculate the new prices from the menu.  They had a time limit.  If they were too slow Freddie the monster who dwelled in the pizzeria would eat them. :P

Afterwards, we reflected on the success of our activities and whether we felt they helped us further develop our neuron pathways for mentally calculating %. 

Making maths hands on and helping children understand why we are learning things in maths is pivotal to student engagement.  Giving them opportunities like this also helps them to develop creative ideas :)