Hello!
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.
[tex] \frac{30}{1} \times \frac{5}{4} = \frac{150}{4} = 37 \frac{1}{2} [/tex]
[tex]\framebox{37 1/2 inches}[/tex]
