On three poles located in a straight line at a distance l = 20 m from one another, electric lamps are suspended at a height h = 4 m above ground level, consuming power N0 = 1 kW each. Find the illumination at a point on the ground under the first pillar if the luminous flux of each lamp is Ф = 15·103 lm. What percentage of the illumination of the earth from the lamp on the first pillar is the illumination from the lamp on the second and third pillars?

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