Given :
The garden is 12m by 10m. 15% of the total area is gardern patch and 3/8 is a flowerbed.
Solution :
Area of lawn = Area of garden - area of garden patch - area of flowerbed.
[tex]A = 12\times 10 - 12\times 10 \times 0.15 - \dfrac{3}{8} \times 12 \times 10\\\\A = 57 \ m^2[/tex]
Total cost of spray is given by :
[tex]Total \ cost =Total\ area \times cost \ per \ area\\\\T = 57 \times \dfrac{0.35}{3} \\\\T = \$6.65[/tex]
Hence, this is the required solution.