Not totally sure about this, but.....
If a : b is 5 : 6 there are 11 equal parts
If b : c is 8 : 9 there are 17 equal parts
And the common denominator is 11 * 17 = 187
So a possible value for b = (6/11) * 187 = 102
But c = (9/8)b = 102 * 9/8 = 114.75
But c needs to be an integer
So...mutiplying this by 4 will give us the integer 459
So....multiplying b by 4 we get 102 * 4 = 408
And a is (5/6)b = (5/6)(408) = 340
So
a , b , c = 340, 408, 459
And a: b = 340: 408 = 5/6 And b : c = 408: 459
So....a + b + c = 340 + 408 + 459 = 1207