3. The sum of the first 20 numbers in the sequence $1,2,3,2,5,2,7,2,9,2, \cdots$ is $\qquad$
【Analysis】The original sequence can be seen as $(1,2),(3,2),(5,2),(7,2), \ldots$ first 20 terms $=1+3+5+\cdots+19+2+2+\cdots+2=10 \times 10+2 \times 10=120$