Give the step-by-step reasoning process and then the final answer. Lizzy's school choir has a mixture of 80 blonde and black-haired girls. Their teacher decides to add 10 more girls to the choir, who turns out to be blonde. If there were 30 blonde-haired girls in the choir initially, how many black-haired girls are present?
The total number of girls in the choir is 80 + 10 = 90 girls. The total number of blondes in the choir after 10 more are added is 30 + 10 = 40 blonde - haired girls. From the total number, the number of black - haired girls is 90 - 40 = 50 black - haired - girls.
The final answer: 50.