Question

A steam-electric power plant delivers 900 MW of electric power. The surplus heat is exhausted into a river with a flow of 2.91×105 kg/s, causing a change in temperature of 1.07 oC.
A. What is the efficiency of the power plant?
B. What is the rate of the thermal source?

MY WORK:
I know that 900 MW is 9x10^8 W. I just am having trouble finding an equation that relates at all to this question... I found a question that was similar but was looking for the temperature change and not the efficiency and tried to re-word their equation but it did not work:
power in watts / efficiency / 4.18 cal/s / flow = change in temp
I got 69.1 as the percent and it did not work, also tried 30.9 and it did not work.

Answer 1

P(lost) = mcΔT/t
From that you can determine efficiency.

Answer 2

How? I don't have mass or time, and I supposed to use 2.91×10^5 kg/s? The problem also does not mention a liquid or anything so how do I know what c to use??