The speed of sound in air depends only on temperature, and it is pretty much the square root of the absolute temperature (degrees C + 273).
So you can construct a formula which will be some constant in knots times the square root of T+273.
I did this for my exams and I think it was 40 x sqrt(T+273), giving 661 knots at zero C.
Edited to add the answer:
0.4 x 40 x sqrt(273-15) = 257 knots so I would go for 250 kts.
Edited again:
I checked my notes and it was 39, not 40, giving 661 kts at 15C,
and so the answer to your question is:
0.4 x 39 x sqrt(273-15) = 250 kts.
Last edited by 24Carrot; 17th January 2012 at 14:14.