Hello!
We can use Faraday's Law of Electromagnetic Induction to solve.
[tex]\epsilon = -N \frac{d\Phi_B}{dt}[/tex]
ε = Induced emf (4.08 V)
N = Number of loops (?)
[tex]\Phi_B[/tex] = Magnetic Flux (Wb)
t = time (s)
**Note: The negative sign can be disregarded for this situation. The sign simply shows how the induced emf OPPOSES the current.
Now, we know that [tex]\frac{d\Phi_B}{dt}[/tex] is analogous to the change in magnetic flux over change in time, or [tex]\frac{\Delta \Phi_B}{\Delta t}[/tex], so:
[tex]\epsilon = N \frac{\Delta \Phi_B}{\Delta t}\\\\\epsilon = N \frac{\Phi_{Bf} - \Phi_{Bi}}{\Delta t}[/tex]
Rearrange the equation to solve for 'N'.
[tex]N = \frac{\epsilon}{ \frac{\Phi_{Bf} - \Phi_{Bi}}{\Delta t}}[/tex]
Plug in the given values to solve.
[tex]N = \frac{4.08}{ \frac{9.44*10^{-5} - 2.57*10^{-5}}{0.0154}} = 914.585 = \boxed{915 \text{ coils}}[/tex]
**Rounding up because we cannot have a part of a loop.
