Answer:
10
Explanation:
Assuming rectangle ABCD, (Because 3 angles are right angled so fourth angle will be 90 by default since sum of internal angles of a quad. is 360)
where,
AD = hypotenuse = 20 (given)
AC = BD = x (given)
Angle ADC = 30 (given)
Therefore,
sin of angle ADC = Perpendicular/hypotenuse = AC/AD
sin 30 = x/20
Since, sin 30 = 1/2, On substituting its value,
1/2 = x/20
x = 20/2
x = 10