GMATinsight
Attachment:
GMATinsight.jpeg
ABCD is a square and points P, Q, R, and S are on the sides AD, AB, BC, CD respectively. Points X and Y are on diagonal of square i.e. BD such that the area of rectangle APXQ is 24 and the Area of rectangle CRYS is 21. If the area of pink shaded square whose diagonal is XY is 9 then find the area of ABCD?
A) 93
B) 95
C) 100
D) 105
E) 110
An educated guess for the answer would be C because it is the only perfect square.
Set \(PX = x\) and \(YR = y\). Then it follows that \(QX = \frac{24}{x}\) and \(YS = \frac{21}{y}\). We have the following equations:
(1) AB = BC, in terms of x and y vertically we have \(x + 3 + y\), and horizontally we have \(\frac{24}{x} + \frac{21}{y} - 3\). These are equal.
(2) XB + XD = YB + YD, then divide the entire equation by \(\sqrt{2}\) to get QX + PX = YR + YS. Which is \(x + \frac{24}{x} = y + \frac{21}{y}\)
It will be hard to isolate one variable at first but if we add the equations we get \(2x + 3 = \frac{42}{y} - 3\) and \(x = \frac{21}{y} - 3 = \frac{21-3y}{y}\).
Plug this in either equation to get an equation with y only, cleaning it up gives \(y^2 + 4y -21 = 0\) and \(y = 3\), \(x = 4\). The side length of the big square is x + y + 3 = 10. Thus the answer is 100, C.
What we can take away from this question is that we would want to use the least amount of variables whenever we can to make solving the system easier.