How about using ratios?:
0.1/5 = 1/50 (making both numbers 10x bigger keeps the ratio the same)
So RDS / (RDS + RL) = 1 / 50
So whatever RDS is; 1 Ohm, 1 kOhm, 1 lot of 270 Ohm, etc.
then (RDS + RL) is 50 of them; 50 Ohm, 50 kOhm, 50 lots of 270 Ohm (ie. 13.5kOhm), etc.
The RDS is one of them, so RL must be 49 of them: in math language, RL is 49 x RDS
As Hevans says, you can use algebra to show that
RL is RDS multiplied by (one less than ( Vdd divided by Vout) )
or rearrange to find any other one of the values.
You can put all the known numbers into a formula which is any way round, then the algebra becomes easier.