17
-19
the top adds what the bottom adds. the top subtracts what the bottom subtracts. the top divides what the bottom divides. the top multiplies what the bottom multiplies.
The number of zeros is determined by how many times 12 divides into 15!. To determine this, since 12 = 22*3, we will look at how many times 2 and 3 divide into 15!. 2 divides 2,6,10,14 once, 4,12 twice, and 8 three times, so that 2 divides 15! exactly 11 times. 3 divides 3,6,12,15 once and 9 twice, so that 3 divides 15! exactly 6 times. From this, we can see that 12 divides into 15! exactly 5 times, since 125=210*35. In other words, the base-12 representation of 15! ends in 5 zeros.
13
Any number adds to 15.
-42
30, 15, 10. 6, 5, 3, 2 and 1.
47
The AVERAGE function.
3
1, 3, 5, 15.
19