Answer:
1. 72000 units. 
2. $19. 
Explanation:
Solution:
Part 1: 
Let's Sort out the data given:
Monthly Cost Fixed = $240,000
Fixed Cost unavoidable = 40% x 240,000
Fixed Cost unavoidable = $96,000
Now, 
Avoidable Fixed Cost will be = $240,000 - $96,000
Avoidable Fixed Cost will be = $144,000
It means that, if the industries obtain products from the outside supplier, it will save or avoid fixed cost of $144,000 per month. 
Now, we also given that, 
Variable Production Cost = $16 per unit 
Purchase Price per unit (Outsider) = $18 per unit 
Increment in Price per unit = $18 - $16 = $2
Hence, 
It will cost the industry an extra of $2 per unit. 
Now, we can calculate the required monthly usage at which it will be indifferent between purchasing and making part MR24. 
Break Even Monthly Usage  = Avoidable Fixed Cost/ Incremental Price per unit. 
Break Even Monthly Usage = $144,000/$2
Break Even Monthly Usage = 72000 units. 
Hence, Monthly usage at which it will be indifferent between purchasing and making part MR24 = 72000 units. 
Part 2: 
Monthly usage as given = 48000 units on which it can avoid the fixed cost of $144,000
Avoidable Monthly fixed cost = $144,000
So, now, we can calculate the avoidable fixed cost per unit as well. 
Avoidable Fixed Cost Per unit = $144,000/48000
Avoidable Fixed Cost Per unit = $3
We also know, 
Variable Production cost per unit = $16 
Avoidable Fixed cost per unit = $3 
So, we can see the maximum purchase price in order to avoid monthly fixed cost. 
Maximum Purchase price per unit = $16 + $3 =$19
It means, $19 is the maximum purchase price, if the industry is approaching the outsider for the monthly usage of 48000 units. It will benefit if the price is less than $19.