Let's imagine the rectangle is in a coordinate system with origin at P. Also, let BR=z. Then, PQ can be expressed as a vector, PQ=(-x,y). Likewise, PR=(10-x,z). Now since QPR is a right angle the dot product of PQ and PR must be zero, so this gives us the following equation:
Notice from this that the maximum value that x(10-x) can take is 25 (indeed, complete the square to find that ). Now, the largest possible value for y is 5 (the point Q can't surpass the side of the rectangle). If y=5, the largest possible value for z is then also 5 (since ), meaning that it's possible to find our point R on the side of the rectangle. Thus y=5, regardless of x, is an acceptable value. So now we have our upper limit for y. Now to find the lower limit.
It's clear that if we decrease y, z increases. We can continue to decrease y all the way until z=5, after that, R surpasses the side of the rectangle, which we don't want. So the lower limit for y must occur when z=5. Going back to the equation I labeled (1), we get that when z=5.
So the values for y for which we can find a point R such that QPR=90 are: