a=log2(36)=2log2(6)=2log2(2)+2log2(3)
log2(3)=(a-2)/2
b=log2(10)=log2(2)+log2(5)
log2(5)=b-1
log2(15)=log2(3)+log2(5)=(a-2)/2+b-1=a/2+b-2