Microbiology calculation - please help!

Hi, I get a lot of questions like this in my genetics practicals and I keep getting them wrong so I know I'm doing the wrong method but I have no idea how to do it and we've never been taught.

The concentration of the nucleotides in a given PCR master mix is 200 uM. How many microlitres (ul) of a 10 mM stock solution do you need to add to prepare 1 ml of mastermix?

I convert it all to SI and do:
(200x10^-6) x (1x10^-3) = 2x10^-7moles of nucleotides in the mastermix

Therefore:
(2x10^-7) / (10x10^-3) = (2x10^-5)L therefore you need 20uL but this is wrong and the correct answer is 50 and I don't see how.

Please help! Thank you.