a). (2.0 x 10^9) x (0.1) = 2.0 x 10^8 cubic meters
b). The edge of a cube with that volume is
(2.0 x 10^8) ^ 1/3 = about 585 metres
(about 0.36 of a mile)
==> I'm pretty sure there are actually more than 2 billion adults in the world, since the total population is well over 7 billion now.