find the sum of first 40 positive integers

Question

Find the sum of first 40 positive integers divisible by 6 also find the sum of first 20 positive integers divisible by 5 or 6.

Ans:          No's which are divisible by 6 are

6, 12 ................ 240.

S₄₀   = 40/2 [6 + 240]

= 20 x 246

= 4920

No's div by 5 or 6

30, 60 ............. 600

20/2 [30 + 600]= 10 x 630

= 6300