no. of structural isomers of methyl anthracene

Getting 3.

Subs. at any of the red C(s) will yield an equivalent structure.
Same goes fr the blue & green ones...

Hmmm yes, u've got a point there.

3 wud be the answer if v were asked the no. of mono-methyl derivatives of Anthracene.

But fr structural isomers toh phenanthrene waale bhi honge, n there'll be lots of open chain structures with their chain isomerism also with the formula C₁₄H₉CH₃ (10⁰ unsaturation)......!!