first off, let's start by changing the percentages to decimal formats.... so let's say we need "x" amount of the 6%, or a solute of 0.06 of "x", and "y" amount of the 2% one, for a solute of 0.02 of "y"
[tex]\bf \begin{array}{lccclll}
&amount&concentration&
\begin{array}{llll}
concentrated\\
amount
\end{array}\\
&-----&-------&-------\\
\textit{butterfat milk 6\%}&x&0.06&0.06x\\
\textit{butterfat milk 2\%}&y&0.02&0.02y\\
--------&-----&-------&-------\\
mixture&100&0.04&4.00
\end{array}
\\\\\\
\begin{cases}
x+y=100\implies \boxed{y}=100-x\\
0.06x+0.02y=4\\
----------\\
0.06x+0.02\left( \boxed{100-x} \right)=4
\end{cases}[/tex]
solve for "x", to see how much 6% buttermilk fat will be needed
what about "y"? well, y = 100 - x
