At an activity level of 4,000 machine-hours in a month, Curt Corporation's total variable production engineering cost is $154,200 and its total fixed production engineering cost is $129,000. What would be the total production engineering cost per machine-hour, both fixed and variable, at an activity level of 4,300 machine-hours in a month? Assume that this level of activity is within the relevant range.

A)$68.33

B)$68.55

C)$70.80

D)$65.86

I don't understand how to calculate variable , and fixed cost.