Draw an embryo sac from memory and label its parts.

Step 1. The cells. The egg, $2$ synergids, $3$ antipodals and $1$ central cell: $1 + 2 + 3 + 1 = 7$ cells.

Step 2. The nuclei. Every cell has one nucleus, except the central cell, which has two: $1 + 2 + 3 + 2 = 8$ nuclei.

Step 3. Fertilisation. The egg nucleus fuses with one male gamete, and the two polar nuclei fuse with the other: $3$ nuclei of the embryo sac take part.