Find the sum of all positive multiples of 3 less than 50 :

Answer: Option C

First multiple = 3
Last multiple (less than 50) = 48
Number of multiples of 3
= $$\frac{48-3}{3}$$ + 1
= 16
Sum :
= $$\frac{16}{2}$$ [2 × 3 + (16 - 1)3]
= 8 × 51
= 408