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Age 14 to 16
Challenge Level 
This problem has two steps. Students may be familiar with factorisation methods for finding divisors but in this situation the numbers do not divide exactly but instead have a common remainder - providing an opportunity for students to look for a step which will allow them to apply something familiar in a new and extending case.
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DOTS Division
Take any pair of two digit numbers x=ab and y=cd where, without loss of generality, ab > cd . Form two 4 digit numbers r=abcd and s=cdab and calculate: {r^2 - s^2} /{x^2 - y^2}.
