(We don't need volume beause heights are the same)
area of box = 6.2 length * 4.9 width = 30.38^2 cm.
area of tray = 31 length * 9 width = 279^2 cm.
Now to find how many boxes fit in the tray.
279^2 / 30.38^2 = 9.1836734693877551020408163265306
(Round it = 9 boxes)
Hope that helps!