A plant manager uses the high/low method to estimate the relation between units produced (per month) and total labor costs (per month). In the mixed cost function, the estimate of b (variable cost coefficient) is $20. For the low month, 100 units were produced while total labor costs were $3,000. Determine the value of a in the mixed cost function. Select one: O a 30 O b. 150 O c. 500 O d. 1,000 O e. 2.000 Of. none of the above