If x/a+b-c = y/b+c-a = z/c+a-b = 5 and a + b + c = 7; the value of x + y + z is : 1. 35 2. 7/5 3. 5/7 4. 42

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If a : b = c : d, prove that :
5a + 7b : 5a - 7b = 5c + 7d : 5c - 7d

If a : b = c : d, prove that :
xa + yb : xc + yd = b : d