Better suggestion. Dont use words. It's a math game. You should be able to showcase what to do with ONLY visuals. Darken out the game on the first missile. Highlight the missile with a focus circle. Then point to one specific equation in the bottom (actually create it for the player). Show that 1 + 5 = 6. Something simple. Then have the missile explode, lighten out the tutorial intro, and let the player continue. Or add a secondary tutorial section with two missiles at once and a multiplication solution.

This also has the benefit that your game becomes internationally-friendly to play (independent of language).

Also, like others have said, dont put in ads. Not for an educational game. Not lucrative at all, and totally goes against what your audience (parents getting this game for their kids) want to see.