With two resistances R1 and R2 we have the 4 following combinations possible:
R1 or R2 if they are used individually.
let R2 be the larger of the two.
R1 R2 / (R1 + R2) if used in parallel.
-- This is the smallest value. It is < R1 and < R2.
R1 + R2 if used in series. This is the largest value. SO it is > R1 and > R2.
Hence, from the given values,
R1 + R2 = 16 Ω
R1 R2 / (R1 + R2) = 3 Ω
So, R1 and R2 are 4 Ω and 12 Ω respectively.