check the given input variables in getSerie
One is an optional agrument

I didn't say that, read the instructions (until you get them) and the sample tests. Do you know what common denominator is?

(207,390)(261,3930)(9,12) // all denominators are different here

You don't need to reduce the fractions after you got the common denominator, the denominator as I already said before, has to be the same for all fractions.

You have to produce a result in the form

(N_1, D) ... (N_n, D)

So you want to say that 2 is same denominator as 12? Seriously?

The output fractions should have the same denominator, as seen in the instructions.

convertFracs [(1, 2), (1, 3), (1, 4)] shouldBe [(6, 12), (4, 12), (3, 12)]