Philip P. answered 11/01/17
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I(d) = 10/d2
I(3) = 10/32 = 10/9 lux
Average rate of change is the slope of the straight line connecting (1,I(1)) and (3,I(3)).
(I(3)-I(1))/(3-1) = (10/9 - 10)/2 = -40/9 lux/m