Question

399+428 make a ten or hundred to add mentally

Answer 1

400+429=829 ..........mma

Answer 2

827

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