20. The variable $a$ can take the values: $-32,-30,-28, \ldots, 2$, $4,6,8$, and the variable $b: -17,-15,-13, \ldots, 11,13$. How many different values can the expression $a+b$ take? Find the product of the largest and smallest values of the expression $a+b$.

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20. Since the first variable can take 21 values, and the second can take 16, the expression $a+b$ can take a total of $21 \cdot 16=336$ different values. The maximum value of the expression $a+b$ is 21 $(8+13=21)$, and the minimum value is $-49(-32+(-17)=-49)$. Their product $(-49) \cdot 21=-1029$.