Question

two resistors R1 and R2 given combined resistance of 4.5 ohm in series and 1 ohm when in parallel resistance

Answer 1

R1+r2 =4.5
And r1*r2/r1+r2=1
R1*r2=1*4.5
R1*(4.5-r1) =4.5
Now solve the equation

Answer 2

R1+r2 =4.5
And r1*r2/r1+r2=1
R1*r2=1*4.5
R1*(4.5-r1) =4.5
Further you can solve it