Hi!
Please take a look at this question. The change in mass of this radioactive isotope can be calculated from this equation. But I don't know the mass of the products. If I had an equation for that, I could have calculated an equation for "loss" in mass (by mass of reactant minus mass of product) that happens in nucluer fission. Then the loss in mass would he converted to energy and I could have attempted calculating the rate using those equations and calculus.
But without any mention of products, I am forced to assume that this equation gives the change in mass of reactant and products combined. Not just change in mass of reactant.
I know it's wrong though.

You're reading too much into it. Assume that the mass lost is completely converted to energy.

Like this?

Like this?

Yes, I think that's correct.

By the way, you could have made the math a bit simpler, as follows:

The beginning amount of mass at t=0 is 0.0085 Kg, So when the mass is half the initial amount there is 0.00425 Kg of material.

dM/dt = -50 * M(t), so when M(t) = 1/2 the initial amount we have dM/dt = -50 *0.00425 = -0.2125 Kg/s. Multiply by c^2 to get the rate of energy production at that instant.

Hey That's a nice shortcut!
Thank-you for everything.