The problem states:
The current through a wire is equal to 100mA. The energy used by the voltage source is 2.5mJ. What is the value of the voltage source?

I=Q/t => Q=It =>Q=(100mA)(1sec) = 100mA
Q=W/V => V=W/Q => V= 2.5mJ/100mA = 25.0mV

I was not sure about subbing in 1sec for t in I=Q/t. My thinking was this was an instant in time. I am not grasping what this problem wants me to see. Is my answer correct, and is there any thoughts about what this problem is trying to show?

 

I think the purpose of the exercise is to make sure you understand the relationship between work and energy.