total work=100%
satish works in 1 day is= 100/18%
biren works in one day 2*100/18%=100/9%
biren and satish together works in 1 day= 100/18 + 100/9%
= 100/6%
together 100/6% done in 1 day
1% done in 6/100 days
100% done in 6*100/100 days
=6days