The answer is 160 because as you stated in the question the number of bacteria doubles every five minutes, and after 10 minutes there are 40. Therefore, after 15 minutes there are 80 (40 doubled is 80), and after 20 (another 5 minutes), the number of bacteria is the double of 80, which is 160. - Hope I helped :) - PeaceLoveHappiness
First you find the lengths of the two vectors, which are both . Then you find the angle given the cos using: cos(∅)= = = 0. Then using ∅=acos(0)= = 90°
