Your hot water tank is insulated, but not very well. To reduce heat loss, you wrap some old blankets around it. With the water at $81^{\circ} \mathrm{C}$ and the room at $21^{\circ} \mathrm{C},$ a thermometer inserted between the outside of the original tank and your blanket reads $36^{\circ} \mathrm{C} .$ By what factor did the blanket reduce the heat loss?