Arjun adds $1 + 2 + 3$ + … and stops as soon as the running total reaches exactly 210. What is the last number he adds?

Explanation

1. 01

   We need $\frac{n(n+1)}{2} = 210$, so $n(n+1) = 420$.

2. 02

   $20 \times 21 = 420$, so $n = 20$.