$18 \times 10 - 30 \times 4 - 20 \times 2 = 20$ 
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 

[a] $$10$$ 
[b] $$20$$ 
[c] $$30$$ 
[d] $$35$$ 
[e] $$40$$ 