Answer:
10
Step-by-step explanation:
(1 +2i)(2 +i) + __ = 5(2 +i) . . . . given
2 +5i -2 + __ = 10 +5i . . . . . . eliminate parentheses
__ = 10 . . . . . . . . . . . . . . . . . . subtract 5i, simplify
The number that goes in the blank is 10.
