A plant has 20 chromosomes in each cell of its root tip…

Step 1. The embryo. It grows from the zygote by mitosis, so its cells have $20$ chromosomes.

Step 2. The endosperm. It grows from the PEN, which is $3n$. Here $n = 10$, so $3 \times 10 = 30$ chromosomes.

Step 3. The perisperm. It is nucellus, a tissue of the mother plant, so its cells are diploid, with $20$ chromosomes.