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## 'Purr-fection' printed from http://nrich.maths.org/

Try to be systematic. How can you narrow down the cases to be tested?

If the last digit of a square number is 9 what can you say about the number?

Can you write down an expression for a number using its digits and powers of 10?

Possible support

Note: Other problems using modulus arithmetic: Dirisibly Yours , Obviously, Rational Round

An article that explains the basic ideas of modulus arithmetic: Modulus Arithmetic and Solution to Dirisibly Yours.