其它
There are $18$ bottles of water in each of $10$ boxes. Six customers want to buy some botlles of water. Four of them buy $30$ bottles of water, and two of them buy $20$ bottles of water. How many bottles of water are left in the boxes 
Overseas Competition->Knowledge Point->Calculation Modules
$18 \times 10 - 30 \times 4 - 20 \times 2 = 20$ 