An allotetraploid plant has its chromosomes derived from two different plants (different chromosomes) and it has 4 chromosomes of each type of chromosome because it received two copies of each chromosome and doubled it. So, this plant, would have 20 chromosomes. (4+6)×2=20
4 -> 2n of one parent
6 -> 2n of the other parent
2 times because it is a "tetraploid" plant and it already has 2 copies of each of its parents chromosomes.