Hollow cylinder heat transfer

Hello, maybe someone here can help me. I am trying to solve the following problem:

A hollow cylinder with outer diameter r_a, inner diameter r_i and a layer thickness s is heated. I measure the surface temperature of the outer wall (t_a), and look for the temperature that occurs inside the cylinder (t_i).

Values: r_a= 51mm , r_i=46mm, s=5mm Lambda =15 W/(mK) (alloy steel) t_a = 50°C t_i= unknown

Can anyone help me? I'm a bit stumped...