Answer
$15 \space ohm$
Work Step by Step
Since, the resistances are connected in series combination.
Formula to calculate the total resistance, we have
$R_{total}=R_1+R_2$
As we are given that $R_1=5 \space ohm$ and $R_{total}=20 \space ohm $
Thus, $20=5+R_2$
$R_2=20-5=15 \space ohm$
