Adam is 5 years older than his sister Barb. The sum of their ages is 21. How old is Adam? How old is Barb?
a) Adam: 13 years, Barb: 8 years
b) Adam: 12 years, Barb: 9 years
c) Adam: 11 years, Barb: 10 years
d) Adam: 10 years, Barb: 11 years