A house is supplied with a 240V a.c mains. To operate a door bell rated at 8v, a transformer is used if the number of turns in the primary coil of the trans...

To calculate the number of turns in the secondary coil of the transformer, we need to use the formula: Np/Ns = Vp/Vs Where Np is the number of turns in the primary coil, Ns is the number of turns in the secondary coil, Vp is the voltage in the primary coil, and Vs is the voltage in the secondary coil. In this case, we know that the voltage in the primary coil is 240V and the voltage in the secondary coil is 8V. We also know that the number of turns in the primary coil is 900. Plugging these values into the formula, we get: 900/Ns = 240/8 Simplifying this equation, we get: 900/Ns = 30 To solve for Ns, we can multiply both sides by Ns: 900 = 30Ns Dividing both sides by 30, we get: Ns = 30 Therefore, the number of turns in the secondary coil of the transformer is 30. So, the correct option is "30".