Question

What is 400 + 20 + 9 in standard form?

Answer 1

400 + 20 + 9 IS standard form

your question is a bit confusing as standard form/expanded form would be like

7232

7000 + 200 + 30 + 2

it is not used to calculate problems

Answer 2

429. If I am correct standard form means to reduce or simplify or even round. So the standard form of it is 430