Final answer:
The equation for Mart's family share cell phone plan cost per member is y = $150 divided by x, where x is the number of lines and y is the cost per member.
Step-by-step explanation:
The equation modeling the situation where Mart and her family split a $150 monthly cell phone share plan equally among its members is y = $150 ÷ x. Here, x represents the number of cellphone lines, and y represents the monthly cost per plan member. Using this equation, we can calculate the individual cost for each family member based on the number of lines they decide to have.