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Expenses
Age 14 to 16
Challenge Level 
What is the largest number which, when divided into each of 1905, 2587, 3951, 7020 and 8725, leaves the same remainder each time?
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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}.
