[tex]a) \frac{x}{3} = \frac{15}{12} \\ 12x = 15 \times 3 \\ 12x = 45 \\ x = \frac{45}{12} = \frac{15}{4} = 3.7[/tex]
[tex]b) \frac{x + 3}{x + 1} = \frac{12}{10} \\ 10(x + 3) = 12(x + 1) \\ 10x + 30 = 12x + 12 \\ - 2x = - 18 \\ x = \frac{ - 18}{ - 2} = 9[/tex]