“总数”和“个数” 分别是 the total amount 和 the number of amounts。

又如：
He gave her a fifth of the total amount. 他给了她总量的五分之一。
I guessed the total amount to be about £50,000. 我估计总额约为5万英镑。
He expects the total amount to be in the low hundreds. 他预计总量会有小几百。
You can get the average by adding these amounts together and dividing the total by the number of amounts. 你可以把这些数加起来用总数除以个数从而得到平均数。