Write 75 (base 10) using base 2 numerals

75 = 64 + 11 = 64 + 8 + 3 = 64 + 8 + 2 + 1 

In base 2, this would be equal to 
1* 2^6 + 1*2^3 + 1*2^1 + 1*2^0

So if you wrote it backwards, it would be:
0123456
1101001

The final answer would be 1001011, because you are just reversing 1101001.