Let us solve first the phenotype of this problem.
Starchy grain = AA (dominant)
Sugary Corn = aa (recessive)
AA x aa = AaAa ; AaAa
A A
a Aa Aa
a Aa Aa
The result of the cross is that the progeny will become heterozygous.
If there is 58 progeny sugary, there will also be 58 starchy. It is 50%.