If (a + b + c + d) : (a + b − c − d) = (a − b + c − d) : (a − b − c + d), prove that a : b = c : d.

Which of the following is the ratio between a number and the number obtained by adding one-fifth of that number?

1. 4 : 5

2. 5 : 4

3. 5 : 6

4. 6 : 5