We can find it using the following formula:
[tex]\begin{gathered} r=\frac{selling_{\text{ }}price-cost}{cost}\times100 \\ where \\ r=40 \\ cost=90 \\ so\colon \end{gathered}[/tex][tex]\begin{gathered} selling_{\text{ }}price=(\frac{40}{100})\cdot90+90 \\ selling_{\text{ }}price=126 \end{gathered}[/tex]