When two resistance $R_1$ and $R_2$ connected in series and introduced into the left gap of a meter bridge and a resistance of $10 \mathrm{\Omega }$ is introduced into the right gap, a null point is found at $60 \mathrm{cm}$ from left side. When $R_1$ and $R_2$ are connected in parallel and introduced into the left gap, a resistance of $3 \mathrm{\Omega }$ is introduced into the right-gap to get null point at $40 \mathrm{cm}$ from left end. The product of $R_1 R_2$ is ______ ${\mathrm{\Omega }}^2$

Hence, the correct answer is 30.