We basically have to find the rent for the period- 1-4-2011 to 31-3-2012
As per first line)) We paid rent of 12000 for 1-10-2010 to 30-09-2011...
Compare this period with the period at the top (highlighted with red). It includes 6 months of that period, i.e 1-4-2010 to 30-09-2011.Therefore, to get the rent of that six months, multiply 12000 by 6/12 whioch equals 6000. Hence 6000 rent paid for 1-4-2011 to 30-09-2011.
As per second line)) We paid rent of 18000 for 1-10-2011 to 30-09-2012..
Again, compare this period with (1-4-2011 to 31-3-2012). It icludes 6 moths, i.e. 1-10-2011 to 31-3-2012. Multiply this with 6/12 too which equals 9000. 9000 rent paid for 1-10-2011 to 31-3-2012.
9000+6000=15000.
Hope you understand. :)