Bunuel
Points Q and R are on segment PS, which has length 24. If PQ is half as long as QS, and QR is 3 times as long as RS, how long is RS?
(A) 2
(B) 3
(C) 4
(D) 8
(E) 12
Sketch a number line, find a common term, and use ratios to move the variables around.
P________Q_______________R___S
1. Take the segments in both ratios and add them to get an idea of what might be a common term.
PQ +
QS = PS
QR + RS =
QS2. Two ratios are given. Both contain QS (the second, indirectly).
Use the more restrictive condition for QS. Here, that's the ratio with more "parts."
3. QS = QR + RS, and \(\frac{QR}{RS}\) = \(\frac{3}{1}\)
Let
RS = x QR = 3xQS (total of ratio parts) =
4x4. Set up the second proportion in terms of x, given that QS = 4x.
\(\frac{QS}{PQ}\) = \(\frac{2}{1}\) = \(\frac{4x}{2x}\)
PQ + QS = PS
2x + 4x = 6x
PS = 6x, and given that PS = 24:
6x = 24
x = 4
5. From above, RS = x, x = 4, hence RS = 4.
Check, starting with first segment PQ: If RS = 4, PQ = 8
That puts the number line values of the variables at
P = 0
Q = 8
R = 20 (QR = 3x = 12, 8+12=20)
S = 24 (RS = 4, 20+4=24)
Correct. RS is 4.
Answer C