Q.How many haploid cells are present in a mature embryo sac?

In angiosperms, each ovule contains one megaspore mother cell (MMC) that undergoes a single meiotic division to give four megaspores, of which normally only one survives to form the embryo sac; so producing 300 seeds (300 fertilized ovules) requires 300 meiotic divisions of megaspore mother cells. On the male side, each pollen (microspore) mother cell undergoes one meiotic division to produce four microspores/pollen grains; since at least 300 pollen grains are needed to fertilize 300 ovules, and each meiosis yields 4 pollen grains, 300 ÷ 4 = 75 meiotic divisions of pollen mother cells are required. Adding the two: 300 (megasporogenesis) + 75 (microsporogenesis) = 375 meiotic divisions in total.

This is a typical Maharashtra HSC Biology question; the underlying idea appears word-for-word in NCERT Class 12 Biology, so CBSE students can use the same reasoning.

In megagametogenesis, a single functional megaspore (the other three degenerate) undergoes three successive mitotic divisions. The first division gives 2 nuclei, the second gives 4 nuclei, and the third gives 8 nuclei, which then organise with cell-wall formation into the typical 7-celled, 8-nucleate embryo sac (female gametophyte) consisting of the egg apparatus (egg cell + 2 synergids), 3 antipodal cells, and a central cell with 2 polar nuclei.

The MSBSHSE textbook and NCERT Class 12 Biology agree on this definition, which is why it turns up in both HSC board papers and NEET previous-year questions.