Quantitative Aptitude :: Numbers - Discussion

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Explanation:
Sum of unit digits of numbers from 1 to 99.
= 10 (1 + 2 + 3 + ..... + 9)
= 10*(9*l0/2)=450
= [1+2+3+....+n=n(n+l)/2]

Sum of ten's digits of numbers from 1 to 99
= 10 (1 + 2 + 3 + ..... 9) = 450
Sum of digits of 100 = 1
Required sum = 450 + 450 + 1 = 901