The answer would be 60.
You have to look closely at the pictures to see that they are not situated with proportional sides laying the same way. If you flip the bottom box so that the 60 is on the bottom, you can line up the proportions.
(80/60) = (40/30) = (x /45)
When reducing the first two fractions you will get a 4/3 ratio.
(4/3) = (x/45)
Cross multiply.
3x = 180
Divide both sides by 3
x = 60