Meritnation Experts,tomorrow is my preboard please help me;i don't understand how to find ratios in the Mendel's Experiment.

The ratios in Mendelian crosses represents the phenotype and genotype of the offspring produced after crossing individuals with different characters in consideration. 
Phenotype means observable characteristics of an individual while as genotype represents genetic make up of an individual.
To understand how ratios are determined, let us take an example of the cross given below:

Inheritance of Two Genes (Dihybrid Cross)

• In dihybrid cross, we consider two characters. (e.g., seed colour and seed shape)

• Yellow colour and round shape is dominant over green colour and wrinkled shape.

​In the above punnet square, it is clear that the offsprings with similar phenotype and genotype are grouped together while determining ratio i.e
Phenotypic ratio − 9:3:3:1 i.e 9(Round yellow) : 3(Round green ) : 3(Wrinkled yellow) : 1(Wrinkled green)

Genotypic ratio -1:2:2:4:1:2:1:2:1 i.e  1(RRYY) : 2( RrYY) : 2(RRYy) : 4(RrYy)​ : 1(rrYY) : 2(rrYy) : 1(RRyy) : 2(Rryy) : 1(rryy)​.