As level maths c3

I tried to do the c) part of this question and the answer of mine turned out be wrong according to the mark scheme. What I did first was differentiate the original formula giving me the rate which was -20e^-0.05-1 and subbed in 50 which gave me -0.604. Where did I go wrong?

That sounds like the right steps, but I'm not sure what you've done to get -20e^-0.05-1. Either it should still have a t in it, or you've substituted incorrectly into the power.

Sorry I forgot to add in the t when typing it but it was in the formula I used as -20e^-0.05t-1

Looks good, but where did that extra -1 at the end come from?

I differentiated so I minused 1 from -0.05t giving me -0.05t-1

Ah, you only do that for polynomials.

To differentiate $e^{g(t)}$ for some function g of t, with respect to t, you combine the chain rule and the property of e that differentiating e^u with respect to u gives e^u. So you end up with $g'(t)e^{g(t)}$

I tried to do the c) part of this question and the answer of mine turned out be wrong according to the mark scheme. What I did first was differentiate the original formula giving me the rate which was -20e^-0.05-1 and subbed in 50 which gave me -0.604. Where did I go wrong?