flex-grow specifies how much of the distributed space a child takes up.
8-1. each section has flex-grow : 0; (default behaviour)
8-2. each section has flex-grow : 1; (number doesn't matter if all the same)
8-3. each section has flex-grow : 1; except 2 which is set to flex-grow : 3; so is three times the size of the others (3:1 ratio)