How many photons are contained in a flash of green light (525 nm) that contains 189 kj of energy?

There are 4.99 × 10²³  photons contained in a flash of green light (525 nm) that contains 189 kJ of energy.

First, we will calculate the energy (E) of each photon using the Planck-Einstein's relation.

where,

Each photon has an energy of 3.79 × 10⁻¹⁹ J. The number of photons that amount to 189 kJ (189 × 10³ J) of energy are:

There are 4.99 × 10²³  photons contained in a flash of green light (525 nm) that contains 189 kJ of energy.

You can learn more about photons here:

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current