what totals are possible and how many variants are there? prove that the middle must be even
prove that 7 and 6 must not be in the same line
ignoring unimportant swaps, show that there are:
two solutions with 2 in the middle
three with 4 in the middle
two with 6 in the middle
prove that only totals of 11, 12 and 13 are possible
why is a total of 14 impossible?
this is the same as a task called 'Haitch'
Graeme Brown produced an Excel interactive version for NRich here