If `a+b=15, b + c =10`, and `a + c = 13`, which of the following is true ?

If a, b, c in R, then which one of the following is true:

If a, b, c in R, then which one of the following is true:

If a, b, c, d are in proportion, then a : b = c : d and written as a : b : : c : d . Which of the following is true?