Here's a classic question on the workings of a transformer:
From the information, we can say Vp = 12 V, Np = 200 and Ns = 40. Putting in the numbers:
This can easily be solved to give Vs = 2.4 V. But since Ns is exactly 5 times smaller than Np, it shouldn't be a surprise that Vs is also 5 times smaller than Vp!
Question: how many turns would she need on the secondary if she wanted a 24 V output?