Question 350559
You almost got it!

The only part you have incorrect is the "If we subtract the second number from half the first number we get twenty" part.

Lets take two variables that will represent numbers.
X is the first number and Y is the second number.
If you take half of the first number, that is the same as saying {{{(1/2)X}}}
Now, we will subtract the second number from {{{(1/2)X}}}
{{{(1/2)X-Y}}}
This expression is equal to 20, therefore, 
{{{(1/2)X-Y=20}}}
This is the algebraic representation of the word problem above.

Hope this helps,
Justin