Question 1169754
Three points (x,y):
(0,50), (20,40), (30,0.05C) and C is for tank's capacity.


* MISTAKE FOUND:  "After further 30 minutes,..."
That meant, 20+30=50 minutes.  
The variable point should be  (50,0.05C).



----------------NO, NOT THIS--------------------
slope, {{{-10/20=-1/2}}};
{{{y=-(1/2)x+50}}}


To find C, use the unknown point.
{{{0.05C=-(1/2)*30+50}}}------------solve this for C.