The cost to buy 2.6 pounds of cheese at the old price was $13.565217...
Explanation
Lets assume, the old price of the cheese is [tex] x [/tex] dollar per pound
Now it is marked up by 15%, so the marked up amount will be: 15% of [tex] x [/tex] [tex] = 0.15x [/tex]
So, the new price of the cheese [tex] =(x+0.15x)= 1.15x [/tex] dollar per pound, which is also given as $6.00 per pound
Then , [tex] 1.15x = 6.00 \\ \\ x = \frac{6.00}{1.15}= 5.217391.... [/tex]
Thus, the old price of the cheese is $ 5.217391... per pound
So, the cost to buy 2.6 pounds of cheese at the old price [tex] = (5.217391... *2.6) =13.565217...dollar [/tex]
