How does seafloor spreading originating at the Juan de Fuca ridge result in the formation of volcanoes like Mount St. Helens and Mount Adams?

Guinea pig coat color is determined by a single gene. the allele for black coat color is dominant to brown. in a cross between twoblack-haired guinea pigs, 20 offspring are born. if both parents were heterozygous, probability would predict that approximately howmany of the 20 offspring would have brown hair?