SOLUTION: how much ground beef with 24% fat should be mixed with 9 lb of ground sirloin that is 6% fat to make a mixture that is 9.6% fat

Let x be the amount of the ground beef with 24% fat to mix (the value under the question, in pounds).


Then your equation is


    fat content + fat content = total fat content,   or


    0.24x       + 0.06*9      = 0.096*(x+9)       (1)


    0.24x + 0.06*9 = 0.096x + 0.096*9


    0.24x - 0.096x = 0.096*9 - 0.06*9


    x =  = one click in Excel spreadsheet = 2.25 pounds of the 6% fat should be mixed.   ANSWER


Check.   Left  side of the equation (1)   0.24*2.25 + 0.06*9  = 1.08 lb.

         Right side of the equation (1)   0.096*(2.25+9)      = 1.08 lb.

         Both sides are equal    <------->  ! Correct !