Let:
[tex]Mark_{\text{ }}up_{\text{ }}Percentage=\frac{Store_{\text{ }}price-Actual_{\text{ }}price}{Actual_{\text{ }}price}\times100[/tex]The store sells the instrument by 30% and the price tag says $104, so:
[tex]\begin{gathered} Mark_{\text{ }}up_{\text{ }}Percentage=30 \\ Store_{\text{ }}price=104 \\ so\colon \\ 30=\frac{104-Actual_{\text{ }}price}{Actual_{\text{ }}price}\times100 \\ 130(Actual_{\text{ }}price)=10400 \\ Actual_{\text{ }}price=\frac{10400}{130} \\ Actual_{\text{ }}price=80 \end{gathered}[/tex]