Check number 19, and 21, and answer number

19) The question is, for which n is n^(4t) equal to 16^(2t)

n^4t = n^(2*2t) = (n^2)^2t

so n^2 must be 16, so n=4

20) Fill in what you know:
 
f(1) = -2f(0)+3 = -4+3 = -1
f(2) = -2f(1)+3 = 2 + 3 = 5


21) The average speed is the total displacement divided by the total time. In this case the total time is easy, it is 5 seconds (from 5 to 10). The displacement will be d(10)-d(5).

d(10) = 0.8*100 = 80
d(5) = 0.8*25 = 20

So average speed is (80-20)/5 = 12