Question

A)5+ 10 +15 +.... +200=

Answer 1

$\displaystyle 5+10+15+...+200=5(1+2+3+...+40)=5 \times \frac{40(40+1)}{2} = \\ \\ =5 \times \frac{40 \times 41}{2} =5 \times \frac{1640}{2} =5 \times 820=4100$