I need a step by step explanation to the following problem in order to make me understand the solution:

Use the following information for Brief Exercises 16-23 through 16-25. CompuWorld Sells two products, R66 and R100, and calculates sales variances using contribution margin. Pertinent data for the current year follow:

	Budgeted				Actual
	R66		R100		R66		R100
Selling Price	50		160		55		155
Variable cost per unit	40		90		43		95
Contribution margin	10		70		12		60
Fixed cost per unit	6		30		5		25
Operating income	4		40		7		35
Sales in units	1,200		400		1,000		1,000

16-23

What is the R100 sales mix variance?

$20,000F

$30,000F

$35,000F

$40,000F

$70,000F