Numbers which are divisible by 5 from 1 to 100 are 5, 10, 15, …………………95, 100.
∴ Sum of the above numbers = 5 +10 + ……………..+ 95 + 100 = 5[1 + 2 + ………………….+ 20]
= 5 [\(\frac{20\,\times\,(20\,+\,1)}{2}\)]
= \(\frac{5\,\times\,20\,\times\,21}{2}\)[∵ Sum of ‘n’ natural numbers = (n(n+1))/2 & n = 20 ]
= 1050
