(4+20i) and (4-5i)

use the distributive property

(4 + 20i)(4-5i)= 4(4-5i) + 20i(4-5i)

get rid of parenthesis

16 -20i + 80i -100i

^{2}i

^{2}=-1 substitute and group like terms then simplify16 + -100(-1) + i(-20 + 80)

116 + 60i