西蒂拥有的钱刚好可以购买 84 瓶装饮料或 126 罐装饮料。她已经花了一部分钱购买了 39 罐装饮料和 42 瓶装饮料。用剩下的钱她还可以再买多少瓶装饮料？ 答：___

1. Find the equivalent number of bottled drinks for the canned drinks bought.

Since the total money can buy $84$ bottled drinks or $126$ canned drinks, the cost of $126$ canned drinks is equal to the cost of $84$ bottled drinks.
Ratio of canned to bottled drinks for the same cost = $126 : 84 = 3 : 2$.
This means $3$ canned drinks cost the same as $2$ bottled drinks.
Number of bottled drinks equivalent to $39$ canned drinks = $\frac{39}{3} \times 2 = 26$.

2. Calculate the total equivalent bottled drinks bought and the remaining she can buy.

Total equivalent bottled drinks bought = $42 + 26 = 68$.
Remaining bottled drinks she can buy = $84 - 68 = 16$.