In angiosperms, syngamy occurs between male and female gametes forming a zygote (n + n = 2n). This is called fertilization.
The second male gamete fuses with the already fused definitive nucleus (which is 2n) and forms the primary endosperm nucleus (2n + n = 3n). This is called vegetative fertilization or triple fusion.
Thus, fusion occurs two times then the whole process is called double fertilization.
This is one of the unique features of angiosperms.
Thus, the endosperm is a triploid structure.
3n = 24 (given), then n = 8. The root cells, however, are diploid (2n). Hence, if endosperm contains 24 chromosomes, then the root cells will have 16 chromosomes.