Question

Find the sum of all 2-digit natural numbers divisible by 5.

Answer 1

The numbers are 10, 15, 20, ..., 95.

Here $$a = 10,\ d = 5, \ l = 95$$. 

Number of terms: $$n = \frac{95 - 10}{5} + 1 = 18$$. 

Sum $$S_{18} = \frac{18}{2}(10 + 95)$$

$$= 9(105)$$

$$= 945$$