1. **Problem Statement:** Find the number of pupils taking 10 to 30 minutes to travel to school on Monday, given that on Tuesday the number was 25% less than on Monday. 2. **Understanding the problem:** Let the number of pupils on Monday be $x$. On Tuesday, the number is 25% less than Monday, so Tuesday's number is $x - 0.25x = 0.75x$. 3. **Given data:** From the problem, Tuesday's number of pupils taking 10 to 30 minutes is 50 (from $150 \div 3 = 50$). 4. **Set up the equation:** $$0.75x = 50$$ 5. **Solve for $x$:** $$x = \frac{50}{0.75} = \frac{50}{\frac{3}{4}} = 50 \times \frac{4}{3} = \frac{200}{3} \approx 66.67$$ 6. **Interpretation:** The number of pupils taking 10 to 30 minutes to travel to school on Monday is approximately 67 pupils. **Final answer:** $$\boxed{67}$$