a,(1+1/51)x(1+1/52)x(1+1/53)x...x(1+1/1963)
=52/51x53/52.54/53x...x1967/1963
=53/51x54/53x...x1964/1963
=18/17x1964/1963
=1964/51
b,(1+1/13)x(1-1/21)x(1-1/26)x...x(1-1/210)
=4/3x20/21x25/26x...x209/210
=80/63x25/26x...x209/210
=1000/819x209/210
=20900/17199