16
There are many combinations of two numbers that add up to 36. One combination is 20 + 16
There are 16C4 = 16*15*14*13/(4*3*2*1) = 1820 combinations.
This question needs clarificatioh. There are 4 one digit number combinations, 16 two digit combinations, ... 4 raised to the n power for n digit combinations.
If order matters and a number cannot be repeated, then there are 16x15x14x13 = 43680 different combinations. If order does not matter then you must divide by the number of ways you can rearrage the same 4 numbers i.e. divide by 4x3x2x1=24 so you can select 4 numbers from a group of 16 in 43680/24 = 1820 ways. In mathematical terms there are 43680 permutations of 16 numbers taken 4 at a time and 1820 combinations of 16 number taken 4 at a time.
Assuming that "number" means digits and that it is permutations (rather than combinations) that are required, the answer is 816 which is 281.475 trillion (approx).
There are 18C4 = 18!/[18-4)!4!] = 18*17*16*15/(4*3*2*1) = 3060 combinations.
16. There are two choices for the first digit (3 and 6). For either choice of first digit there are two choices for the second digit, and so on. 2 x 2 x 2 x 2 = 16.
If you allow for zeros, that's 10^16, which is ten quintillion, or 10,000,000,000,000,000. If not, it's 9^16, which is 1,853,020,188,851,841, or 1.85 quintillion. (1.85x10^15)
16
If they can repeat, then: 17^6=24,137,569 If they can't repeat, then: 17*16*15*14*13*12=8,910,720
double 16