Two things:

A) When working out the second equation i read the requirement as how long the fuel left would last and came up with

t = c/b time = currentFuel/burnRate

t = 80 / 2 = 40

by reworking the 1st equation you are working out how long we’ve been flying for aren’t you?

t = (-c + m ) / b

t = (-80 + 100) / 2 = 20 / 2 = 10

[Starting with c as 0 you happen to get an answer of 50 for time on max fuel…]

B) In manipulating the equations you expanded -(b * t) to -b * -t but to me that gives a different answer if b is 2 and t is 10 the former gives -20 and the latter +20

(you also divide two negatives and come up with a negative -100/-2 = -50 where i think that is 50)

Apologies if I’m wrong - its early Saturday and brain hasn’t been used in a while.

If I am right you need the guys to properly review and I need my money back…and/or a few free courses