Which harmonics will be missing when duty cycle = 30%?
If the duty cycle is a rational number

$$D=\frac{p}{q}$$

and \$p\$ and \$q\$ have no common factors, then all harmonics with indices \$n=kq\$ (\$k=1,2,\ldots\$) are zero. So for a duty cycle of \$30\$% \$p=3\$ and \$q=10\$ and all harmonics with indices that are multiples of \$10\$ are zero.