Bunuel
A took 15 seconds to cross a rectangular field diagonally walking at the rate of 56 m/min and B took the same time to cross the same field along its sides walking at the rate of 72 m/min. What is the area of the field?
(A) 46 m²
(B) 50 m²
(C) 54 m²
(D) 64 m²
(E) 72 m²
Solution:
We see that the diagonal of the rectangular field is 56 x 15/60 = 56 x 1/4 = 14 meters long. We see that the total length of the two dimensions (length and width) of the field is 72 x 15/60 = 72 x 1/4 = 18 meters long. Now, if we let L = the length of the field, then 18 - L = the width of the field, and we can create the equation (using the Pythagorean theorem):
L^2 + (18 - L)^2 = 14^2
If we let A = area of the field, then A = L(18 - L). Adding 2A, or 2L(18 - L), to both sides of the equation above, we have:
L^2 + 2L(18 - L) + (18 - L)^2 = 14^2 + 2A
[L + (18 - L)]^2 = 196 + 2A
18^2 = 196 + 2A
324 = 196 + 2A
128 = 2A
64 = A
Answer: D