Systems of Linear Inequalities, Word Problems - Examples

Cat has a science test and a math test that she needs to study for. Studying a chapter of math takes an hour and a half. Studying a chapter of science takes half an hour. She has a little under $40$ hours free in the week to study and has to spend at least $15$ of them studying. Math is Cat's worst subject so she wants to study at least twice as much math as she would science. Write a system of linear inequalities to represent this problem.