15. Find the sum: $1+2+3+\ldots+1999$.
15. Let $S=1+2+3+\ldots+1999$. Then: $S=1999+1998+\ldots+3+2+1$ and $2 S=(1+1999)+(2+1998)+\ldots+(1999+1)=2000 \cdot 1999$. Therefore: $S=1000 \cdot 1999$. Answer: 1999000.