Answer:
120
Step-by-step explanation:
If there is a monohybrid cross that involved genes that obeyed Mendelian inheritance this means that parental generation includes two true breed parents (both homozygous but with different genotype).
If we name the gene with A:
P: AA x aa
F1: Aa Aa Aa Aa
F2: AA Aa Aa aa
Half of them will be heterozygous, it means 120.