820 bytes is the data which comes to network layer of P. at network layer 20 more bytes of IP header is added.
at P-R1 max MTU is 1024-DLL header = 1012 bytes. this includes IP header also. we need to fit 820 bytes in 1012-20 bytes
but, as 992>820 no need to do fragmetation
R1-R2 contains 256-8 is max MTU= 248 B. this includes IP header also. max data it can support is 228 bytes
but 228 is not divisiible by 8
so divide 820 4 parts.. 224,224,224,148.. each of these packet gets 20 bytes IP header+ 8 bytes DLL header.
R2-Q max MTU is 500 bytes. no need to do fragmentation
1)224+20+8
2)224+20+8
3)224+20+8
4)148+20+8
total 932 bytes