1. **State the problem:** Miss Lee had $12.50 more than Miss Mak initially. After Miss Lee spent $18, Miss Mak had 3 times as much money as Miss Lee had left. We need to find how much money Miss Lee had at first. 2. **Define variables:** Let $L$ be the amount of money Miss Lee had at first. Let $M$ be the amount of money Miss Mak had at first. 3. **Write equations from the problem:** - Miss Lee had $12.50 more than Miss Mak: $$L = M + 12.50$$ - After Miss Lee spent $18, she has $L - 18$ left. - At that time, Miss Mak still has $M$ (she did not spend any money). - Miss Mak had 3 times as much money as Miss Lee had left: $$M = 3(L - 18)$$ 4. **Substitute $M$ from the first equation into the second:** $$M = L - 12.50$$ So, $$L - 12.50 = 3(L - 18)$$ 5. **Solve the equation:** $$L - 12.50 = 3L - 54$$ Bring all terms to one side: $$L - 12.50 - 3L + 54 = 0$$ $$-2L + 41.5 = 0$$ $$-2L = -41.5$$ $$L = \frac{41.5}{2} = 20.75$$ 6. **Interpret the result:** Miss Lee had $20.75 at first. **Final answer:** $$\boxed{20.75}$$