Answer
See below.
Work Step by Step
If we multiply both sides by $R\cdot R_1 R_2 R_3$ we get: $R_1 R_2 R_3=R R_2 R_3+R_1 R R_3+R_1 R_2 R=R(R_2 R_3+R_1 R_3+R_1 R_2)$.
Thus $R=\frac{R_1 R_2 R_3}{R_2 R_3+R_1 R_3+R_1 R_2}$.
Here this is $R=\frac{(5)(4)(10)}{(4)(10)+(5)(10)+(5)(4)}=\frac{200}{40+50+20}=\frac{20}{11}$