Ingeneuer: (40-j10)x(20-j10)
j is the same as i. Use i in the calculator and also put the coefficient of i in front of it (and use * for multiplication):
(40-10i)*(20-10i)
and you'll have it. Or, by hand:
= 800-200i-400i+(10i)^2
= 800-600i+100i^2
= 800-600i-100
= 700-600i
