Problem 20

C₉H₁₀
Rule 2, omit O, gives C₇H₁₄
9 - 10/2 + 1 = 5 degrees of unsaturation.
Look for an aromatic ring, plus another pi bond or aliphatic ring.

The bands at 3000-2850 indicate C-H alkane stretches. The band at 3060 indicates C-H aromatic stretch; aromatics also show bands in the regions 1600-1585 and 1500-1400 (C-C in-ring stretch), and 900-675 (C-H out-of-plane).

This is the structure. See if you can assign the peaks on your own.

Again, the integral values need to be multiplied by two to give the right number of Hs.