is a rational number because it can be put as a fraction. AnIrrational numbers cannot be a fraction since they are continuous (unless you estimate/round them up)
You have to multiply 1 1/4 by 30 because each math book is 1 1/4 in. thick, and 30 books is 1 1/4 by 30. First change 30 and 1 1/4 to an improper fraction.