Q=mLf Q=1000 and Lf=79.8 c/g what is m? How do you come to the conclusion of this formula equation?

If Q = mLf, then m = Q/(Lf) (when Lf is not zero). (You don't need to know it is a thermodynamics problem to understand that this is true about equations.)

Also, it looks like you left the units of Q out of the statement of the problem. Depending on what the units of Q are, you will get different answers.
Also, future posts should be posted in Homework help, and show more of the problem, and your attempts to solve them.

If Q = 1000 calories, then m = (1000 calories)/(79.8 calories/gram) =(1000/79.8)grams = 12.5 grams.

But if Q = 1000 Joules, then m = (1000 Joules)(1 calorie/4.184 Joules)/(79.8 calories/gram) = 3.00 grams.

This question is from textbook "Strategies for problem solving"

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