You can solve this by setting up a system of equations. Let b represent the bikini and s represent sunglasses, then write two equations that go with the situation.
b + s = 110
s + 100 = b
To make this easier, I am going to solve the first equation for b and then set the two b values against each other in an equation.
b + s = 110 Subtract b from both sides
b = 110 - s
Now, set the two s values equal to each other and solve for b.
110 - s = s + 100 Add s to both sides
110 = 2s + 100 Subtract 100 from both sides
10 = 2s Divide both sides by 2
5 = s
Since s = 5, we know that the sunglasses are $5.
