1600^2=2,580.000 * 10 = 25,600,000
4700^2=22,090,000 * 6 = 132,540,000
then sum
25,600,000 + 132,540,000 = 158,140,000
now take the square root of 158,140,000 = 12,575.3 Lm
your correst for 16 light bulds
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so thats also 10 light...