Question

how much low fat milk (1%) fat and how much whole milk (4%) fat should be mixed to make 5 gal of reduced fat mild 2%

Answer 1

Method 1 using Algebra:
Let x=volume of 1% milk in gallons
then (5-x)=volume of 4% milk

5*2%=x*1%+(5-x)*4%
Solve for x:
10=x+20-4x
3x=10
x=3 1/3 gal. (1% milk)
5- 3 1/3 = 1 2/3 gal. (4% milk).

Method 2 (mental calculations)
Mentally put the 1% and 4% on a linear scale as follows:
1%..................................4%
The target percentage is 2%:
1%......<b>2%</b>..............4%
Take the difference:
.....1.............2.........
these are the ratio of the <b>4%</b> and <b>1%</b> milk respectively (note the inversion of position), i.e.
4% milk : 1% milk = 1:2
Therefore
volume of 4% = 5*1/(1+2)=1 2/3 gal
volume of 1% = 5*2/(1+2)=3 1/3 gal.