Question

All the numbers from 1 to 99 are multiplied together. Use a pattern to determine the last digit of the product. Justify your answer.

Answer 1

1 = 1 = 1!
1*2 = 2 = 2!
1*2*3 = 6 = 3!
1*2*3*4 = 24 = 4!
1*2*3*4*5 = 120 = 5!
1*2*3*4*5*6 = 720 = 6!
...
I assume that 99! will be even and will also end in a 0

Answer 2

since 99! = 1*2*3*4*5*...
2*5 is a factor, so 99! is a multiple of 10, ending in zero.