The sum of the last four digits of Marian’s phone number is 30. How many such four-digit sequences are there?
This is a partitions problem in 4 parts:
9 + 9 + 9 + 3 = 30...........4 permutations
9 + 9 + 8 + 4 = 30...........12
9 + 9 + 7 + 5 = 30...........12
9 + 9 + 6 + 6 = 30...........6
9 + 8 + 8 + 5 = 30...........12
9 + 8 + 7 + 6 = 30...........24
9 + 7 + 7 + 7 = 30............4
8 + 8 + 8 + 6 = 30...........4
8 + 8 + 7 + 7 = 30...........6
If you add up all the permuations, you should get =84
