Using algebra to solve for the the cost of the glass lenses gives the value for cost of glass lenses to be 4.
The cost of the glass lenses is calculated as follows:
Let cost of glass be y
Cost of plastic lenses = 4y
Cost of eye exam and frames be z.
z + y = 30 (i)
z + 4y = 42 (ii)
(ii) - (i)
3y = 12
y = 4
Inconclusion, the cost of the glass lenses is 4.
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