I visited the pet store last week and found two kinds of pets for sale: rabbits for $5 and parakeets for $9. If I had $14 to spend, I could buy one rabbit and one parakeet. If I had $24 to spend, I could buy three rabbits ($15) and one parakeet ($9), which would amount to exactly $24. However, if I had $13 to spend, no combination of rabbits and parakeets would cost $13. Assuming that money is no concern, what is the largest amount of money that I could have and not be able to spend all of it on some combination of rabbits and parakeets?

Starting from number 25, number 26 is a possibility, but then you get number 31 which is larger. Then the following numbers all show a possible combination:

N° 9's 5's

25 0 5

26

27 3 0

28 2 2

29 1 4

30 0 6

31 - -

32 3 1

33 2 3

34 1 5

35 0 7

36 4 0

37 3 2

38 2 4

39 1 6

40 0 8

41 4 1

42 3 3

43 2 5

44 1 7

45 0 9

A pattern starts to show 35-39 ; 40-44 and so on.