P = 5n - 200
where
P: profit
n: number of pairs of earrings sold.
To find how many pairs of earrings must Lena sell to earn a profit of $450, we have to replace P = 450 into the equation and solve for n, as follows:
450 = 5n - 200
200 is subtracting on the right, then it will add on the left
450 + 200 = 5n
650 = 5n
5 is multiplying on the right, then it will divide on the left
650/5 = n
130 = n
Lena must sell 130 pairs of earrings to earn a profit of $450.
