electricity

We know that when the resistance are connected in series and by question we get

When connected in parallel, by question  we get

From equation 1 and 2 we get two values of  and that is

 

let us take

Then from equation 1 we get

R+r=90, R=90-r, putting this value in equation 2 we get

when r is 60 ohms then R is 30 ohms and when r is 30 ohms then R is 60 ohms.