Answer:
(a) Planet A: 1,000,000,000
Planet B: 10,000,000
Planet C: 100,000,000
[10^n will always equal a 1 followed by n zeroes. So 10^9 was a 1 followed by 9 zeroes which equals 1,000,000,000 or 1 billion]
(b) Dwarf Planet: 2 x 10^6
[Multiply the radius by 2 to get the diameter. So the dwarf planet's diameter is 2/100 of Planet C's diameter. You then multiply the 2/100 by Planet C's diameter to get 2,000,000 or 2 x 10^6