Respuesta :
60 ways because the easy way to solve this problem is identify the numbers being used and than multiply them by each other.
Slot method
20 options
3 slots
First slot: 20 options
Second slot: 19 options (one used up for 1st slot)
Third slot: 18 (one less than last)
Multiply them
20 times 19 times 18=6840
Answer is 6840 ways