The sum of all the natural numbers between 100 and 1000 which are multiples of 5 is

The natural number between 100 and 1000 are $$105, 110, 115......995$$

It is making an arithmetic series.

Sum of the terms of arithmetic series $$S_n=\dfrac{n(2a+(n-1)d)}{2}$$

$$t_n=a+(n-1)d$$

$$995=105+(n-1)5$$

$$n-1=\dfrac{995-105}{5}=178$$

$$n=178+1=179$$

Hence, the required sum $$S_n=\dfrac{179(2\times 105+(179-1)\times 5)}{2}$$

$$S_n=\dfrac{179\times(210+178\times 5)}{2}=550\times 179=98450$$

Hence, the required sum $$S_n=98450$$