2. (10 points) Calculate: $1+2+4+5+7+8+10+11+13+14+16+17+19+20=$
【Analysis】By observation, it is found that using the commutative and associative laws of addition to combine the front and back numbers can result in a total of 21, with a total of 7 pairs, i.e., $(1+20)+(2+19)+(4+17)+(5+16)+(7+14)+(8+13)+(10+11)$, and the calculation can be made accordingly.

【Solution】Solution: $1+2+4+5+7+8+10+11+13+14+16+17+19+20$
$$
\begin{array}{l}
=(1+20)+(2+19)+(4+17)+(5+16)+(7+14)+(8+13)+(10+11), \\
=21 \times 7 \\
=147
\end{array}
$$

Therefore, the answer is: 147.