If (x + 3) : (4x + 1) is the duplicate ratio of 3 : 5, find the value of x.

If m : n is the duplicate ratio of m + x : n + x; show that : x² = mn.

If x, 2, 10 and y are in proportion, the values of x and y are respectively :

1. 0.2 and 0.25

2. 0.2 and 50

3. 0.4 and 50

4. 0.4 and 25

If x : y = y : z, then x² : y² is :

1. 1 : x

2. x : y

3. x : z

4. z : x