A mixture contains two mutually inert solutions $’X’$ and $’Y’$ in equal volumes. The mixture is examined in a spectrophotometer using a cuvette. It is observed that the transmittance is $0.40$. With only the solution $’X’$ in the same cuvette, the transmittance is $0.20$. With only solution $’Y’$ in the cuvette the transmittance is __________.