Isolating selling price,
[tex]\begin{gathered} \frac{\text{ markup \% }}{100}\text{= }\frac{\text{ selling price - cost}}{\text{ cost}} \\ \frac{\text{ markup \% }}{100}\cdot\cos t=\text{selling price - cost} \\ \frac{\text{ markup \% }}{100}\cdot\cos t+cost=\text{selling price} \\ \cos t(\frac{\text{ markup \% }}{100}+1)=\text{ selling price} \end{gathered}[/tex]The markup is 75 %. Supposing that the cost of a T-shirt is $6.00, then the selling price will be:
[tex]\begin{gathered} \text{selling price = 6.00}\cdot(\frac{75}{100}+1) \\ \text{selling price = 6.00}\cdot(0.75+1) \\ \text{selling price = 6.00}\cdot1.75 \\ \text{selling price = \$}10.5 \end{gathered}[/tex]