Answer:
$4.60
Step by Step:
First, you have to subtract $20 from $6.20 which is the change you got back. Once you remove that, you will get $13.80, the actual amount of all three pairs of socks. Now, you have to figure out how much each pair of socks is. So you need to divide 13.80 by 3 which is $4.60. So each pair of socks was $4.60
Hope that helps :D