The answer given was "0,52*114,9 + 0,48*238,3 = 174,1 mmHg" (the commas were Euro-style). This assume a completely ideal solution of the components A and B. Solutions are rarely ideal to nearly the precision given in the answer. The reason is, to put it in crude terms, that A and B don't stick to each other exactly as well as each sticks to itself. As a result, the vapor pressure of the solution is almost always noticeably higher or lower than the simple weighted average.
There's a nice discussion of the rule that they used and the deviations from it on http://en.wikipedia.org/wiki/Raoult%27s_law.
One very familiar example of a solution with strong deviations from the simple rule is the water-ethanol combination, described here: :http://www.chemguide.co.uk/physical/phaseeqia/nonideal.html
I'm being a little picky here. The reason is that in thermodynamics there are some amazing exact relations, so it's important not to get various rules-of-thumb mixed up with the exact laws. One of the surprising exact laws in this specific area (vapor pressure of solutions) is discussed on another of our answers.http://van.physics.illinois.edu/qa/listing.php?id=1470
(published on 01/19/2011)