I have milk that contains 1 percent fat and milk that contains 4 percent fat. a customer wants a double latte made with 1/3 of a pint of 2 percent milk. how much of each type of milk should i use ?

You should use 2/9 of a pint of the 1% milk and 1/9 of a pint of the 4% milk.

Let x be the amount of 1% milk used.  1% = 1/100 = 0.01; this gives us 0.01x to represent this.

We know that we will use the 4% milk to make up the rest of the 1/3 pint; this gives us 0.04(1/3-x).

Together, they will make 1/3 pint of 2% milk, or 0.02(1/3)

This gives us the equation 
0.01x+0.04(1/3-x) = 0.02(1/3)

Multiplying, we have
0.01x+0.04/3-0.04x = 0.02/3

We can multiply everything by 3 to eliminate the fractions:
0.01x*3 + 0.04 - 0.04x*3 = 0.02
0.03x + 0.04 - 0.12x = 0.02

Combining like terms, we have
-0.09x + 0.04 = 0.02

Subtract 0.04 from both sides:
-0.09x + 0.04 - 0.04 = 0.02 - 0.04
-0.09x = -0.02

Divide both sides by -0.09:
-0.09x/-0.09 = -0.02/-0.09

This gives us x = 2/9, so we will use 2/9 of a pint of the 1% milk.

1/3 - 2/9 = 3/9 - 2/9 = 1/9; we will use 1/9 of a pint of the 4% milk.