Maximum Circles on Rectangular Field Diagonals

XAT 2020 · Quantitative Ability & Data Interpretation · Hard · Quantitative Aptitude

A rectangular field is 40 meters long and 30 meters wide. Draw diagonals on this field and then draw circles of radius 1.25 meters, with centers only on the diagonals. Each circle must fall completely within the field. Any two circles can touch each other but should not overlap. What is the maximum number of such circles that can be drawn in the field?

  1. A.

    38

  2. B.

    39

  3. C.

    36

  4. D.

    37

  5. E.

    40

Answer

D

Explanation

Diagonal length = 402+302=50\sqrt{40^2 + 30^2} = 50 m. For a circle of radius r=1.25r = 1.25 m touching two adjacent sides of the corner (angle θ\theta at corner where tan⁡θ=30/40=3/4\tan \theta = 30/40 = 3/4), sin⁡(θ/2)\sin(\theta/2) determines distance from vertex to center = 1.25×1+(3/4)2/...=1.2521.25 \times \sqrt{1 + (3/4)^2}/... = 1.25 \sqrt{2} roughly. Usable diagonal length is 50−2×0.51=48.9850 - 2 \times 0.51 = 48.98 m. Max circles per diagonal = ⌊48.98/2.5⌋=19\lfloor 48.98 / 2.5 \rfloor = 19. Since 1 circle can be placed exactly at the intersection of the two diagonals (common to both), each half diagonal gets 9 circles. Total circles = 4×9+1=374 \times 9 + 1 = 37.

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