A solution to this question can be done as follows:
Q units cost $9 for parts and $15 for labor, or $24 cost/unit
R units cost $10 for parts and $20 for labor, or $26 cost/unit
the sale price of Q units is $150, so the per unit income is $150 - 24 = $126/unit
the sale price of R units is $175, so the per unit income is $175 - 26 = $149/unit
Based on that analysis, the manufacturer should build ONLY R units. Combining Q and R units will result is a lesser income.