Hi, can you please explain how they got the answer to the below question with working out and formula to use

Maxie Pty Ltd makes and sells two types of shoes, Plain and Fancy. Product data is as follows:

Plain:

Unit Selling Price $20

Variable cost per unit $12

Fancy:

Unit Selling Price $35

Variable cost per unit $24.50

Sixty per cent of the sales in units are Plain and annual fixed expenses are $45 000 and the sales mix remains constant. How many units of Fancy must Maxie Pty Ltd sell to earn a target profit of $31 500?

Answer: 3,400