Expanded form- 300,000+4000+1 . In words, Three hundred and four thousand one.
5n - 10 is the way to write the expression you wrote in English above.
It does not mean much until we set it equal to something.
For example:
The difference of five times a number n and 10 is 25.
Now
5n - 10 = 25
by subtracting 10 from each side we get 5n=35 and then if we divide both sides by 5, we get n=7.
Solve for n by simplifying both sides of the equation, then isolating the variables.
n=54
Hope I helped