Question

3 multiply by what is equal to 30 plus what

Answer 1

Call the hidden variable here "x"

3x = 30 + x
3x - (30 + x) = 0
3x - 30 - x = 0
2x - 30 = 0
x = 30 : 2 = 15

Recheck : 3 x 15 = 30 + 15 -> 45 = 45 (true)