Cross out three digits in the number 51489704 so that the resulting number is divisible by 45. What number is left?

The factors of 45 are 9 and 5.

A number divisible by 45 needs to be divisible by both 9 and 5.

For a number to be divisible by 9 the sum of the digits must also be divisible by 9 and the number needs to end with either a 0 or a 5 to be divisible by 5.

This means the number needs to end with the 0 so cross out the last number (4)

Now find a sum of 5 remaining numbers divisible by 9.

5 + 1 + 4 + 8 + 0 = 18 which is divisible by 9

Cross out 9,7 and the last number 4 to get the number: 51480

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