The number is five hundred three thousand four hundred seven and seven hundred thousand?

I think they are looking for something like twenty-five and six equals thirty-one.

I might be a smart alec and if they asked for nine AND one, I'd write 8. which is 9 logical and with 8. They can also be talking about two numbers such as 25 and 6. I think the word should be plus otherwise it's up to interpretation.

The number is five hundred **insert comma** three thousand four hundred seven ** insert comma ** and seven hundred thousand?

If I've interpreted your initial ramble, I believe what you are looking for in an answer is something about 703.907

What I came up with is 503 407 and 700 000 which makes 1 203 407.

The number is five hundred three thousand four hundred seven and seven hundred thousand?

How about?

500 3000 407 700,000 or

503,407 and 700,000