They are: 31/1, 31.0, 031 and XXXI
17
There are 32C3 = 32*31*30/(3*2*1) = 4960 combinations. I do not have the inclination to list them all.
There is only one number that is equal to 15, and that is 15.
There are no two real numbers that will add to -9 and multiply to 81. Using complex numbers, the two numbers are: (-9/2 + i9/2√3) and (-9/2 - i9/2√3)
Four examples of irrational numbers are 21/2, 31/2, 51/2 & 71/3
31 and 3
31 = 1 + 2 + 3 + 4 + 21
30 1/3 31 1/3 32 1/3
They are: 7+11+13 = 31
Multiplied, 31 and 3 Divided, 279 and 3 Added, 90 and 3 Subtracted, 96 and 3
31 and 2
If by "equal up" you mean add up, there are at least two answers: (3, 7, 13, 31) and (5, 7, 11, 31).
13+7+6+5=31
The factors of 279 are: 1 3 9 31 93 279These pairs of numbers, when multiplied, equal 279:(1, 279)(3, 93)(9, 31)
31 and 2
1 x 279, 3 x 93, 9 x 31
3 and 31