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Given that
n+2n+3n+....+99n
= n*(1+2+3+........+99)
= (n*99*100)/2
= n*99*50
= n*9*11*2*25
To make it perfect square we need 2*11
So n = 2*11 = 22
Now n2 = 22*22 = 484
So number of digit in n2 = 3