Find a Number B < 61 Such That the System $\begin{aligned} 3 x+20 y & =15 \\ 9 x+b y & =45\end{aligned}$

Find a number b < 61 such that the system of linear equations has infinitely many solutions.
$\begin{aligned} 3 x+20 y & =15 \\ 9 x+b y & =45\end{aligned}$