Pranav, the question is to find radius of the highest orbit assuming Bohr's model holds true.  according to electron filling rule... 2, 8, 18, 32, (remaining = 40).  we can see that principle quantum number is 5 counting these terms. 

And in writing radius in Bohr's model, we just need n and Z. 

Hope you understand it.Let me know if you have further doubt.