Question

下面这个表达式的值是多少？ $1 + 2 - 3 + 4 + 5 - 6 + {}$ $7 + 8 - 9 + 10 + 11 - 12$

What is the value of the following expression? $1 + 2 - 3 + 4 + 5 - 6 + {}$ $7 + 8 - 9 + 10 + 11 - 12$

$18$

$21$

$24$

$27$

$30$

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按三项一组计算： $(1+2-3)$ $+(4+5-6)$ $+(7+8-9)$ $+(10+11-12)$ 。这四组的和分别是 $0,3,6,9$ ，所以总和是 $0+3+6+9=18$ 。

Group the expression as $(1+2-3)$ $+(4+5-6)$ $+(7+8-9)$ $+(10+11-12)$ . These group sums are $0,3,6,9$ , respectively, so the total is $0+3+6+9=18$ .