A4. Marija travels to school by a combination of walking and skateboarding. She can get there in 38 minutes if she walks for 25 minutes and skateboards for 13 , or in 31 minutes if she walks for 11 and skateboards for 20 . How long (in minutes) would it take her to walk to school?
Solution 1: Denoting her walking speed by $w$, skateboarding speed by $s$, and distance to school by $D$, we have
$$
D=25 w+13 s, \quad \text { and } D=11 w+20 s .
$$

Subtracting these gives $7 s=14 w$, and so $D=51 w$. Hence, it will take 51 minutes.
Solution 2: Decreasing the skateboarding time by 7 minutes increases the walking time by $14=2 \times 7$ minutes and the total time taken by 7 minutes. So decreasing the skateboarding time by 13 minutes will increase the walking time by $26=2 \times 13$ minutes and the total time by 13 minutes, from 38 to 51 .
Answer: 51 .