pK_ln, of bromocresol green is 4.7. Thus, ratio of its yellow and blue forms at pH 3.7 is

$$\begin{gathered} \mathrm{pH}={\mathrm{pK}}_{\text{In }}+\log \frac{\text{ [blue] }}{\text{ [yellow] }} \\ 3.7=4.7+\log \frac{\text{ [blue] }}{\text{ [yellow] }} \\ \Rightarrow -1=\log \frac{\text{ [blue] }}{\text{ [yellow] }}\Rightarrow 1=\log \frac{\text{ [yellow] }}{\text{ [blue] }} \\ \therefore \text{ [yellow] : [blue] }=10:1 \end{gathered}$$