Focusing on what appears to be a misunderstanding (rather than the pressure level calculations etc.)...
Originally Posted by LH2 and the other posters saying much the same
....oh, by the way, the correct answer is FL50, because it asks for the first available flight level. FL30 is not available for cruising for the reasons explained above by other posters.
This has been done in the past - many times. ICAO refers to Transition Level being the first available FL above Transition Altitude. The Transition Layer is the airspace between the Transition Altitude and Transition Level - it is of variable depth and depends on the Transition Altitude, prevailing pressure and, in some cases, national rules. In the context of Transition Altitude/ Level/ Layer, ICAO makes no reference to IFR separation or other ATC issues.
The interpretation of the word 'available' varies. To some it means the first 'proper' flightlevel that exist above Transition Altitude, to others it means the first that exists above a minimum Transition Layer of 500ft, yet others want a minimum Transition Layer of 1000ft. All may be valid interpretations if there is no laid down standard for a particular State or piece of airspace.
I've seen some States set a Transition Level 2 or 3 thousand feet above the Transition Altitude so that ATC do not have to worry about separation between aircraft flying at altitudes and those at flight levels unless the QNH is unusually high (oh I do hope I got tht the right way round!). I presume such States do not have traffic flow problems and can afford to lose two or three levels......but that's really another issue.
In the UK the rules are not very clear - it's a while since I looked in detail but a uick scan of the relevant docs suggests not much has changed - but if you look in the AIP ENR 1.7 para 3.3 it says
The vertical position of aircraft at, or below, any Transition Altitude will normally be expressed in terms of Altitude. The vertical position at, or above, any Transition Level will normally be expressed in terms of Flight Level. When descending through the Transition Layer the vertical position will be expressed in terms of Altitude, and when climbing in terms of Flight Level. It should not be assumed that separation exists between the Transition Altitude and the Transition Level.
This clearly indicates that, in the UK, Transition Level is expected to be the first 'proper' FL that exists above the Transition Altitude. It may well be that some pilots want, for some reason, to ensure that there's a certain minimum vertical distance between themselves at a FL and those who choose to fly below Transition Altitude and avoid flying at the true Transition Level. It's also a fact that some ATC units/individual controllers will only allocate a minimum FL that provides a Transition Layer of at least 1000ft (whether it's necessary for separation purposes or not) - but this is not a requirement.