I submit that it's not a matter of "unable" to give the answer, but that it takes time after an unusual request to do the math.
Some pilots may try to overcomplexify the calculation. I would rather simplify it as much as possible: I know that at landing weight a 744 can cruise or hold at just about any altitude, in any configuration, at 2,500 Kg/hr/engine, or 10 T/hr or less. That makes calculations simple and conservative. If I'm at my [sometimes] minimum fuel over destination of 7T, I have about 45 minutes to flameout.