The answer is 11111111100.

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225 = 9 * 25

Numbers divisible by 25 end in 25, 50, 75 or 00. So our number must end in 00.

Numbers divisible by 9 have a digital root divisible by 9. So our number must have 9 1's in it.

Therefore, the number in question is 9 1's followed by 00, or 11,111,111,100.