10100101
L=1010 & R=0101
now first copy R as a L of Result i.e 0101
and apply function F on R with Key which is given XOR in this case so,
0101 XOR 1100 = 1001
now again XOR this with L of input i.e.
1001 XOR 1010 = 0011
Hence final answer will be 01010011