I think you have misinterpreted the question,
It provides a condition where r can be said to be combined resistance in a parallel circuit.
The following statement explains what the word combined means here. If you'll translate the words into an equation you'll see that
\(1/r = 1/x + 1/y\)
Hope this helps. Hit kudos, It motivates.
saransh2797
Hey Bunuel,
If the question has mentioned that the combined resistance is r, then ofcourse mathematically r = x + y.
Please clarify

Thanks
Bunuel
Shiv2016
BunuelWhy are we not considering r= x+y as it is mentioned that r is the combined resistance? Is it because we don't know how they are connected.
The question clearly gives the relationship between r, x, and y and it is NOT r = x + y:
the reciprocal of r is equal to the sum of the reciprocals of x and y.