Transformers Questions

The primary and secondary coils of a transformer have $50$ and $1500$ turns respectively. If the magnetic flux $\varphi$ linked with the primary coil is given by $\varphi ={\varphi }_0+4t$, where $\varphi$ is in weber, $t$ is time in second and ${\varphi }_0$ is a constant, the output voltage across the secondary coil is:

A transformer is used to light a $100\text{W}$ and $110\text{V}$ lamp from a $220\text{V}$ mains. If the main current is $0.5\text{A}$, the efficiency of the transformer is approximately: