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40% of 70 = 70% of 40

On first inspection, the prompt seems rather trivial and easy to confirm. Yet, it has proved to hold a fascination for younger secondary school students who can verify its truth but are rarely certain that the relationship will hold for "more complicated" statements of the same type (including for percentages greater than 100). After a the period in which students ask questions and make observations about the prompt, the inquiry often involves a fast-paced period of exploration with students testing different types of numbers. Once students have explained why the prompt is true, they can extend their reasoning by comparing other statements (below) to the original prompt. 
70 increased by 40% is the same as 40 increased by 70%.   
Although the additional statement is false if "same as" is taken to refer to the
outcomes, the increases (28 in the case of the prompt) are the same. It follows that the gaps between the two starting numbers (40 and 70) and the outcomes (68 and 98) are also the same. A proof (right) that if the outcomes are the same, then the two starting numbers (a and b) must be the same is accessible to students as early as year 7 (grade 6).
 40 increased by 70% is the same as 70 decreased by 40%.  
This second additional statement is also false, but it has in the past led to an inquiry to find two numbers that would make such a statement correct. The relationship between the two numbers is shown on the right. One example that arises regularly during this inquiry is: 25 increased by 50% is the same as 50 decreased by 25%. Once the general relationship is found, pairs of numbers can be generated. For example, 30 increased by 75% is the same as 75 decreased by 30%. Students can then plot values of a and b on a graph (or use graphing software) in order to explore the relationship further.
40% of 70 = 70% of 40
51% of 640 = 34% of 960
This additional statement invites students to use proportional reasoning. What do the two statements have in common that mean they are both true? The support sheet below invites students to find more pairs of the same type.
The inquiry has developed into finding fractions of amounts (and fractions of fractions) when students change the prompt. Why does 5/8 of 2/3 = 
2/3 of 5/8? The prompt has, furthermore, led to students trying to express their observations algebraically.

You can read the questions and observations about the prompt from two year 8 mixed attainment classes here.

Lesson notes

Promethean flipchart    download
Smartboard notebook   download

Adapting the prompt
An important feature about a prompt is that it is set just above the understanding of the class to arouse curiosity and generate conjectures without intimidating students by being too 'difficult'. The prompt, then, must take into account the prior learning of the class. One secondary maths department decided to use the percentages prompt with all their setted year 8 classes. Teachers were concerned that the prompt would be too easy for the higher attaining classes, but too difficult for students with the lowest prior attainment. With these concerns in mind, the prompt was changed in the following ways to provide enough intrigue at each level:
10% of 50 = 50% of 10
40% of 70 = 70% of 40
47% of 74 = 74% of 47
20% of 30% of 40 = 40% of 30% of 20  
The lower attaining students explored examples involving multiples of 10 guided heavily by the teacher; the higher sets regulated their activity with the cards and finished with students presenting their proofs for the conjecture that "the order does not matter."
Year 8 students start the inquiry