[tex]x-\ the price \ of \ the \ gift \ \\ \\ 10\% \ of \ x= \frac{10}{100}*x= \frac{1}{10}*x=\frac{x}{10}- the \ discount \\ \\x^{(10} -\frac{x}{10}= \frac{10x-x}{10}= \frac{9x}{10} -the \ new \ price \ \\ \\4*9.00 \$=36 \$ -the \ price \ after \ discount \\ \\ \frac{9x}{10}=36 =>x=\frac{10*36}{9} \\ \\ \boxed{x=40 \$}[/tex]
the variant A.$40.00
