In this case, we'll have to carry out several steps to find the solution.
Step 01:
Data:
x = quarts of milk (2% fat)
y = quarts of milk (4% fat)
x + y = 9
0.02x + 0.04y = 0.24
Step 02:
system of equations:
x + y = 9 eq.1
0.02x + 0.04y = 0.24 eq. 2
eq.1:
x + y = 9
x = 9 - y
eq.1 in eq.2:
0.02(9 - y) + 0.04y = 0.24
0.18 - 0.02y + 0.04y = 0.24
0.02y = 0.24 - 0.18
0.02y = 0.06
y = 0.06 / 0.02 = 3
x = 9 - y
x = 9 - 3
x = 6
The answer is:
Nathan used twice as much 2% milk as 4% milk.