Multiplying with a Complex number, using only the imaginary part
I have a number I need to multiply with a changing complex number, using only the imaginary number.
Say I have a number 25, and a complex number of 20+5i. I need a formula that would in this case multiply 25 with 5, giving me a value of 125. The complex number is a changing one, so I need something that does this accurately and not a cheap cheat.
Also, having the formula for the use of real numbers instead would be useful too for me in the future.