Correct Answer: Option C
ধরি, কলম সংখ্যা=x
শর্তমতে, 240/x-240/x+1=1
or,240(x+1)-240x/x(x+1)=1
or, x2+x-240=0
or, x2 + 16x -15x -240 =0
or, (x+16)(x-15) = 0
x=15
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