Function that separates a list in two other ones

[buran]

80% of 12 is 9.6, therefore ([1, 2, 3, 4, 5, 6, 7, 8, 9], [10, 11, 12]) is correct

well, under all math rules 9.6 would round to 10 and i guess that is why OP expects list with len 10 :-)

I also guess OP is using pythin2 thus / is actually floor division.