Three unlinked genetic loci, each with two alleles, determine the differences in sugar content (and therefore sweetness) in a diploid fruit. These genes contribute equally to sugar content, and their effects are additive. The least sweet variety (aabbcc) and the sweetest (AABBCC) are bred to produce an F1 generation. All the F1 plants produce fruits of intermediate sugar content. Phenotypes are listed below: Plants Sugar content value, relative to sweetest plant
aabbcc 40%
AABBCC 100%
F1 70%
The fruit breeder's goal is to sample a variety of sugar contents. She breeds F1 plants with one another to produce F2 progeny. For each sugar content listed below, what is the probability that an F2 plant will produce fruit with that sugar content?
a. 70%.
b. 50%.
C. 80%.
d. 100%.

This is actually a case of additive effect that means, it doesn't matter which allele is dominant which is recessive. Here what really matter is the total number of dominant and recessive alleles.

Offspring with_

All 6 dominant allele possess 100% sugar content.

any 5 dominant alleles  possess 90% sugar content.

Any 4 dominant alleles  possess 80% sugar content.

Any 3 dominant alleles  possess 70% sugar content.

Any 2 dominant alleles  possess 60% sugar content.

Any 1 dominant allele  possess 50% sugar content.

Zero dominant allele (aabbcc)  possess 40% sugar content.

Hence by this concept we can easily find the percentage of sugar content of any offspring.

what is the probability that an F2 plant will produce fruit with that sugar content?

a. 70% - 17/64. (all Blue coloured offsprings)

b. 50% - 5/64. (all green cloured offsprings)

c. 80% - 15/64. (all red coloured offsprings)

d. 100% - 1/64 (single purple colored AABBCC offspring).

Parents AABBCC X aabbcc

              (100%) x (40%)

F1 AaBbCc

(70%)

F1 selfing AaBbCc x AaBbCc

Possible gametes of AaBbCc are ABC, ABc, AbC, Abc, aBC, aBc, abC, and abc.

Cross shown to draw a Punnett Sqaure-