Answer and Explanation:
The computation of the opportunity cost that should be divided to accomplish the lesser opportunity cost is shown below:
For Gary
The Opportunity cost of 10 hours of calculation is 
= 10 hours of slides 
 Now the opportunity for 1 calculation hours would be 1 hour of slide
Thus, the opportunity cost of 1 hour of calculation = 1 hour of slide 
 For Diane: 
 The Opportunity cost of 12 hours of calculation would be
= 20 hours of slide preparation 
 So for one hour it would be
= 20 ÷ 12 
= 1.67 hours of slide preparation.