Answer:
ou have I=200mA, E=40J, t=30s, and you want to find the voltage drop.
First, you should know that P=V⋅I , so V=PI
Second, you have the amount of energy converted in a certain amount of time, so E=P⋅t
So, find the power and use it to find the voltage drop.
this works , but i thought energy was defined by W = P * t whitch would then be P = W/t