Problem 2.
=========

The first mistake made by many is to use the numbers given for N-P-K
without converting to fractions. For example, 

1.19 - 0.38 - 0.14    is converted to fractions  1.19/100 - 0.38/100 - 0.14/100

before being used in the equations. 

The first equation should be the balance equation for Nitrogen (N):

	1.19/100*x1  15.3/100*x2 + 1.22/100*x3 = 5

Answer to (d):  100*5/(x1+x2+x3) for each of N-P-K, because 5 lb is the amount of
each ingredient and x1+x2+x3 is the total number of pounds of material (about 425).

Problem 3.
=========

There were many typos in Lab 5 problem 3. They have been corrected by the time you read this. 
Please get a new PDF copy at the course web site and read the edits made, which are documented
after the Lab 5 click-button.

The first equation is constructed from 

   x1' = C_i * r_i  -  C_o * r_o  (Edwards-Penney brine tanks, 7.3)

using

   C_1 = 1 gram/liter,  r_i = 4 liter/min
   C_o = x1(t)/(volume V1)= x1(t)/20, r_o=r_i=2+2=4

Then the first DE is

   x1'(t) = 4 - (4/20) * x1(t)