Question

Solve if you can
3+6+9+........ . +90

Answer 1

This is an arithmetic progression
a=3
d=3
l=90
l=a+(n-1)d
90=3+(n-1)3
n-1=29
n=30
Sum of terms
=n/2 (a+l)
=30/2 (3+90)
=15*93
=1395

Answer 2

3(1+2+3...........+30)

$= 3 \times \frac{30 \times 31}{2} \\ = 3 \times 15 \times 31 \\ = 15 \times 93 \\ = 1395$