Solution

40198

First name
Maths Challenge Group
School
Moor Park Primary
Country
Age
10

We could see that the smallest triangle was an eighth of the whole shape, and that if you put the 2 medium triangles together they would half fill the square, therefore they must be a quarter each. That meant an eighth plus a quarter plus a quarter makes 5 eighths leaving the largest triangle as 3 eighths of the square.