we will assume that the 20 sections can be red, blue or white only.

let

r = the number of red sections

b = the number of blue sections

w = the number of white sections

the probability of spinning red is 20% that is

r/20 = 0.20 multiply both sides by 20

r = 0.20*20

r = 4 red sections

the probability of spinning blue is 1/2

b/20 = 1/2 multiply both sides by 20

b = (1/2)*20

b = 10 blue sections

w = 20 - (r + b)

w= 20 - (4 + 10)

w = 6 white sections

there are 6 white sections.