I need the equation for substitution or elimination process of this problem. Ryan works at the donut shop where he makes $10.25 per hour. he also works part time at the school book store where he makes $8.75 per hour. If he worked 20 total hours last week and made $196, how many hours did he work at the book store? I ONLY NEED THE EQUATION

Aclassroom is made up of 11 boys and 14 girls. the teacher has four main classroom responsibilities that she wants to hand out to four different students (one for each of the four students). if the teacher chooses 4 of the students at random, then what is the probability that the four students chosen to complete the responsibilities will be all boys?