Genetics problem-Height of a plant
From the source that Amory cites :


  You know that the number of offspring is shown by Pascal's triangle,
  right?
  
  For two alleles you have 1 : 2 : 1
  
  For three alleles 1 : 3 : 3 : 1 
  
  For four 1 : 4 : 6 : 4 : 1 etc. 
  
  So, if 4 in 1000 are 12 and 36 cm
  tall, that means that every in 250 plants is tall like that. And on
  which level of Pascal's triangle you have sum of 250? For two alleles:
  1 + 2 + 1 = 4 For threee: 1 + 3 + 3 + 1 = 8 For four: 1 + 4 + 6 + 4 +
  1 = 16 etc. until 1 : 8 : 28 : 56 : 70 : 56 : 28 : 8 : 1 Here you see
  that you have 9 options of height. The difference is 36 cm - 12 cm =
  24 cm. 24 cm /9 = 3 cm, so 1 plant will 12 cm, 8 plants will be 15 cm,
  28 plants will be 18 cm, 56 plants will be 21 cm, 70 plants will be 24
  cm (i.e. "the majority of F2 are like P1 and F1") and 56 plants will
  be 27 cm tall out of 250 plants. Thus C is the closest.