Importance of 100%

Here is another question that my students always have problem with.

Allan had 40% more money than Ben. Charles had 20% more money than Allan. When Charles spend $1680, his amount of money decreased by 50%. Allan and Ben each spent 20% of their money. How much money did three of them left altogether?

The key for this question is to let Ben be 100% and be careful with Charles' percentage.

        84% --> $1680
        1% --> $20
        Total 276% --> $5520

The common mistake is the highlighted part - finding the percentage for Charles. 

Students normally just add 20% to Allan's 140% to find Charles. But this is wrong because Allan is not 100%.

**Note: We can only add the percentage directly when the base is 100%