You need to find has many ways 3 numbers goes between 1-20. Create a table to insert the number in. Right down all the different numbers/ways. Although, no numbers can be followed by another number e.g. 1, 2, 3, 4 or 1,4,9,10….. must be 1, 3, 5, 8 0r 1,6,8,13 so no number follow.
Example:
1 3 5
1 3 6
1 3 7
1 3 8
1 3 9
1 3 10
1 3 11
1 3 12
1 3 13
1 3 14
1 3 15
1 4 6
1 4 7
1 4 8
1 4 9
1 4 10
1 4 11
1 4 12
1 4 13
1 4 14
1 4 15
1 5 7
1 5 8
