Resposta:r=4

Explicação passo a passo:

K=20--->n=K+2---->n=20+2--->22,a1=3,a22=87,r=?

an=a1+(n-1).r

87=3+(22-1).r

87=3+21.r

87-3=3-3+21.r

84=21.r

r=84/21

r=4

PA(3,7,11,15,19,23,27,31,35,37,43,47,51,55,59,63,67,71,75,79,83,87)