If the highest common factor and the lowest common multiple of a, b and c are 2019 and 20190 respectively, find the sum of possible value(s) of a.

The prime factors of 20190 are 2*3*5*673

Now 3*673 = 2019, so this is a, say.

b and c must be multiples of this, so b 2*2019 = 4038 and c = 5*2019 = 10095

The three numbers are: 2019, 4038 and 10095

I'll leave you to add them up.