By using only two resistors, r1 and r2, a student is able to obtain resistances of 3 ω, 4 ω, 12 ω, and 16 ω. the values of r1 and r2 (in ohms) are:

The values of r1 and r2 are 4 ohms and 12 ohms.
Putting 4 ohms and 12 ohms in parallel gives 3 ohms.
Putting 4 ohms and 12 ohms in series gives 16 ohms.