You start by cross multiplying:
9 x 8 = 72
This is considered the fraction on the left's value.
You do the same for the other side:
10 x 7 = 70
This is considered the value of the fraction 7/8
72 has a greater value than 70.
Therefore, 9/10 is larger.
Let the equal sides of the isosceles Δ ABC be x.
Given that the perimeter of Δ ABC = 50m.
Therefore, 2x + AC = 50 --- (1)
It is also given that the perimeter of Δ ABD = 40m.
Therefore, x + BD + AD = 40
BD is the median of the Δ ABC. Therefore, D is the midpoint of AC.
So AD = CD.
Or, AD =
AC
Therefore, 
Multiply both sides by 2.
2x + 2BD + AC = 80
From (1), 2x + AC = 50.
Therefore, 2BD + 50 = 80
2BD = 80 - 50
2BD = 30
BD = 15m.
The answer is 6. Because 4x6=32