Batteries were sold in packs of 9 batteries and 5 batteries. Adam bought 12 packs with a total 88 batteries. How many packs of 5 batteries did Adam buy?

1. Assume all packs bought were 9-battery packs and calculate the total number of batteries.

Assume all $12$ packs are $9$-battery packs.
Total batteries $= 12 \times 9 = 108$

2. Find the difference between the assumed total and the actual total number of batteries.

Difference in total batteries $= 108 - 88 = 20$

3. Find the difference in the number of batteries between the two types of packs.

Difference per pack $= 9 - 5 = 4$

4. Calculate the number of 5-battery packs by dividing the total difference by the difference per pack.

Number of $5$-battery packs $= 20 \div 4 = 5$