Atransformer has 20 turns in its primary coil and 60 turns in its secondary coil. the input voltage for the transformer is 25 volts. what is the output voltage?

As, the transformer works on the principle of mutual induction between two coils.So, according to Faraday's law,
               induced emf=-number of turns in coil *[change in flux per unit time] 
write above equation in formula form,
                               V= -N[ΔФ/Δt]  
Now, write the above equation for both primary and secondary coils of transformer and equate them(as ΔФ/Δt remains same for both of them):
we get the relation:
                                           V_{s} / V_{p}=N_{s} / N_{p}
s stands for secondary coil.
p stands for primary coil.
 By putting the values:
                                          V_{s}/25=60/20

                                           V_{s}=75volts (output voltage).