mcNugget Numbers
McDonald's sells Chicken McNuggets in boxes of 6, 9, or 20. Obviously one could purchase exactly 15 McNuggets by buying a box of 6 and a box of 9. Using only combinations of boxes of 6, 9, and/or 20 McNuggets,
a. Could you purchase exactly 17 McNuggets?
b. How would you purchase exactly 53 McNuggets?
c. What is the largest number for which it is impossible to purchase exactly that number of McNuggets?
d. Let's say you could only buy the McNuggets in boxes of 7, 11, and 17. What is the largest number for which it is impossible to purchase exactly that number of McNuggets?

Related External Resources 
Runsums
Find the smallest sum of consecutive groups of numbers.
Runsums 2
Find combinations of numbers that produce certain sums.

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