Answer:
827
Step-by-step explanation:
Hi there!
For any addition and subtraction, if you add and subtract 1 (or any other number), the result remains the same. In this case
399 + 428 = 399 + 428 + 1 - 1
Reordering
(399 + 1) + (428 -1)
Which gives
400 + 427
Which can easily solve to get 827
