We have:

`1/4=1/10+1/R_2`

We need to multiply throughout by the **lowest common
denominator: 20***R*_{2}

`1/4=1/10+1/R_2`

`(20R_2)/4=(20R_2)/10+(20R_2)/(R_2)`

`5R_2=2R_2+20`

`3R_2=20`

`R_2=20/3=6 2/3Omega`

"`Omega`" is the symbol for "ohms", the unit of resistance.