Maths: Algebra: Confused with '2' working out time

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 :wink:

Hi @EddieRocks, you’re absolutely right and I’ll get the video re-recorded today to fix these errors.

A) The question should be “how long have we been flying” and not “how long can we fly”.
B) The answer is correct but the method is inaccurate/unclear so needs fixing.

I’ll be re-recording all of the audio for this lecture today, as listening back now you can clearly tell that I was suffering with a wicked head cold at the time (which is probably how the errors occurred in the first place).

Apologies for allowing these mistakes to slip through the net and make it into the course, and thank you as always for your feedback. It’s very much appreciated so keep it coming.

@EddieRocks, this video should now be patched. Please do keep the feedback coming, it is genuinely appreciated and the course will change based on your comments / corrections.

The question now makes more sense and working out the solution looks better too now.

1 Like

Okay, so I got:

adjust max fuel so that it lasts 120 seconds until running out:


m = c+bt,
therefore m = 0 + 2*120,
m = 240;

adjust burn rate:


b = (m-c) / t,
b = (100 - 0) / 120,
b = 100/120,
b = 0.8333333333333333333333333333333333333333333 forever and ever

1 Like