Well we know that 1 inch is 2.54 cm. But how wide is .18? Lets do .18 x 2.54 to get 0.4572. Now lets add 2.54 + 0.4572 to get 2.9972. This means that the maximum width of the aorta is 2.9972 cm. Hope this helped :)
It doubles every 4 hours, so after 24 hours there will be 6 doublings. 2^6 = 64 So, the population will be (3,000,000 * 64) or 192,000,000 in 24 hours. The answer is B.