Explanation
When crossing an organism with genotype Aa (heterozygous) with one that has genotype aa (homozygous recessive), the offspring can inherit either the A or a allele from the Aa parent and only the a allele from the aa parent.
This results in a 1:1 ratio of genotypes, with half of the offspring being Aa (heterozygous) and the other half being aa (homozygous recessive).
The incorrect ratio of "1/2 Az & 1/2 aa" mentioned in the question is likely a typo.