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 a : b = 2 : 1, the value of (7a + 4b) : (5a - 2b) is :

1. 9 : 4

2. 4 : 9

3. 11 : 3

4. 3 : 11