Year 0--# of New Bulbs=2500; # aged 1=0; # aged 2=0
Year 1--# New= 2500*.2=500; # aged 1= 2500*.8 =2000; # aged 2=0
Year 2--# New=.2*500 +.25*2000=600; # aged 1= 500*.8=400; # aged 2= .75
* 2000=1500;
Year 3--# New=.2*600 + .25*400 + .5*1500 = 970
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