For this problem, you have to find two unknowns. The price rate for each pack of battery denoted as x, and the amount of packs of batteries he can buy with his $5. So, we need to formulate two independent equations in order for the degrees of freedom to be zero. However, there are more unknowns than equations:
2x=y
z = 5/x
where y is the total amount he paid for the two packs of batteries, and z is the number of packs of batteries he bought out of $5. Technically, this problem is unsolvable. But, for educational purposes, let's just assume one of those variables that is likely to be given. Suppose two packs of batteries costs $2.4. Then,
2x = 2.4
x = 1.2
Each pack of battery costs $1.2. The number of batteries he could buy with $5 is
z = 5/1.2 = 4.167
At most, he could only buy 4 packs of batteries.