Sorry ma'am but i couldn't get it.
I am having some doubts...
Just consider the case where r1 is 000100 and r2 is 01010, now r will be 00010001010.
But r is not satisfying the given condition of the problem i.e. number of 0's is divisible by 5 and number of 1's is even as in this case there are eight 0's and three 1's..
I don't know whether my doubt is relevent or not?