Question about Level I economics Ch 17 - Question 3 (MRP)

In a given firm, skilled workers currently produce twice as much of its product as unskilled workers do per hour worked.  Skilled workers earn $20 per hour, and unskilled workers earn $8 per hour.  Based on this information, the firm should:

The answer is  C: Increase the use of unskilled workers and/or decrease the useof skilled workers. 在看答案解释的时候有点不明白为什么firm should try to equate hourly output with hourly wage for both types of workers?  To maximize profits, wage must equal a worker's MRP，这里是不是只一个企业的average wage should be equal average workers' MRP.

多谢！