What would be the number of chromosomes of the aleurone cells of a plant with $42$ chromosomes in its root tip cells?

Root tip cells are diploid ($2n=42$).Therefore, $n=21$.Aleurone layer is a part of the endosperm, which is triploid ($3n$) in angiosperms.Number of chromosomes in aleurone cells $=3\times 21=63$.