1. **State the problem:** We need to find Mona's present age given that Sana is 18 years older than Mona and that after 6 years, Sana will be twice as old as Mona. 2. **Define variables:** Let Mona's present age be $x$ years. 3. **Express Sana's present age:** Since Sana is 18 years older, Sana's age is $x + 18$. 4. **After 6 years:** Mona's age will be $x + 6$ and Sana's age will be $(x + 18) + 6 = x + 24$. 5. **Set up the equation:** After 6 years, Sana will be twice as old as Mona, so $$x + 24 = 2(x + 6)$$ 6. **Solve the equation:** $$x + 24 = 2x + 12$$ $$24 - 12 = 2x - x$$ $$12 = x$$ 7. **Conclusion:** Mona's present age is $12$ years.