Everyday maths - 400*5+12*2

Cod attempted to learn every day, but failed.
This year, Cod is back again, perhaps not struggling with laziness and procrastination.
If you see me missing for 48 hours, you can post any question to frighten me for the day so the study do not stop.

I shall try to post one question every day in the following 52*7 days, without any solution. Of course, besides question for the day, everyone is welcomed to post. (But this is not a homework thread -.-)

No specific topics or difficulty are set for the questions, but try to add interesting questions or solutions to here.

Solution should only be posted after 12 hours so we can leave time for the others to solve.

Questions for Day 1 and 2 of the year:

I think a is 4 and b could be 12 or something, idk

err sadly nope

hmm something went wrong (from 3rd to the 4th)
well you got it correct because it becomes 1 later…

and for the second picture your L.H.S. equations look missing the square, but nice try

The coordinates of the midpoint: the mean of the coordinates of both endpoints
so:
(b+1)/2 = -1
b = -3

(-4+4)/2 = a
a = 0

Here is the graph

Question for Day 3:
For 3 persons A, B and C, it takes 10 days, 15 days and 20 days respectively for them to finish a project alone. If they work on the project together, and when working on the project, A rest for 1 day, B rest for 3 days, C do not take a rest, how many days are required for the project to be finished?