I posted the answer earlier. This is just additional explanation for anyone interested! :)
a+b = 10
b+c = 20
a+c = 24
a + 20 - b = 24
a - b = 4
a = 4 + b
4 + b + b = 10 -> 2b = 6 -> b = 3
a = 10 - 3 = 7
c = 20 - 3 = 17
a = 7
b = 3
c = 17
a+b+c = 27 kg