You are bargaining with a salesperson for the price of an item. Your first offer is $a$ dollars and theirs is $b$ dollars. After you raise your offer by a certain percentage and they lower their offer by the same percentage, you arrive at an agreed price. What is that price, in terms of $a$ and $b$?
1. Let \( x \) be the percentage (expressed as a decimal) by which you raise your offer and the salesperson lowers their offer. 
2. Your new offer after raising it by \( x \) percent is \( a + ax = a(1 + x) \).
3. The salesperson's new offer after lowering it by \( x \) percent is \( b - bx = b(1 - x) \).
4. At the agreed price, these two amounts are equal:
   \[
   a(1 + x) = b(1 - x)
   \]
5. Solve for \( x \):
   \[
   a + ax = b - bx
   \]
   \[
   ax + bx = b - a
   \]
   \[
   x(a + b) = b - a
   \]
   \[
   x = \frac{b - a}{a + b}
   \]
6. Substitute \( x \) back into the expression for the agreed price:
   \[
   \text{Agreed price} = a(1 + x) = a \left(1 + \frac{b - a}{a + b}\right)
   \]
7. Simplify the expression:
   \[
   a \left(1 + \frac{b - a}{a + b}\right) = a \left(\frac{a + b + b - a}{a + b}\right) = a \left(\frac{2b}{a + b}\right) = \frac{2ab}{a + b}
   \]

The final answer is \(\boxed{\frac{2ab}{a + b}}\)