Problem

Source: Hong Kong TST - HKTST 2024 1.4

Tags: algebra



After expanding the polynomial $(1+x+y)^{2023}$ and collecting the like terms, we obtain the expression $$a_0+a_1x+a_2y+a_3x^2+a_4xy+a_5y^2+\cdots+a_ky^{2023}$$Find the total number of $a_i$'s which are divisible by $5$.