As part of the class's unit on genetics, mr. franks' 8th period science class decided to cross two plants. each of the parent plants had red flowers and the genotype rr. if the plants produce 100 offspring, about how many of the plants will have red flowers?

R (red colour) is clearly dominant, because both parental strains are red in the heterozygous form. In a punnet square, you get a ratio of 1(RR- red):2(Rr-red):1(rr- not red). It is easier to think of these in percentages for this question: 25% (RR) = red. 
50% (Rr) = red.
25% (rr) = not red. 

They have made the question easier by using 100 offspring. Therefore, you can just convert the percentages to numbers. 

Therefore: 
Because RR and Rr are both red, add 25 + 50 = 75.

75 offspring should be red.