You can think of this problem in basic maths.
You have 4 pairs of socks sold at 4.19$ and you want to buy only three.
So in order to know the price of 3 pairs, you should first calculate the price of one pair of socks.
use cross multiplication to get the price of one pair as follows:
price of one pair = 4.19 / 4 = 1.0475$
Therefore,
price of 3 pairs = 1.0475 x 3 = 3.1325$
