I have read the rules.
I tried to solve this using the equation for the area of constant drying speed: t = K (X2 - X1), where K is Ls/(ARc), Ls is mass of dry material [kg], R is speed of drying [kg of water / (h m^2)], A area of drying [m^2]
the problem is that I can't figure it out how to determine X2 and X1 from the data given. the key is to determine X at 60°C and X at 90°C.
As for the second part of the problem, I have no idea at all how to try to solve it