Question

 A transformer with 40 turns on the high voltage winding is used to step down the voltage  from 240V to 120V. Find the number of turns in the low voltage winding.

Answer 1

Using the expression for the transformation ratio

$k=\frac{U_1}{U_2}=\frac{N_1}{N_2}$ Where $U_1=240V,U_2=120V,N_1=40$

write down

$N_2=\frac{N_1 \cdot U_2}{U_1}=\frac{40 \cdot 120}{240}=\frac{40}{2}=20(turns )$