Tanya ug no 6. Gimana caranya ya

Misal :
ayam kampung = a
ayam potong = b

maka :
perbandingan jumlah : a : b = 5 : 8
a-b = 21
ditanya, a+b = ?

a/b = 5/8
5a = 8b
a = 8b/5

a-b = 21
8b/5 - b = 21
8b - 5b = 105
3b = 105
b = 35

a = 8b/5
   = 8(35)/5
   = 56

a+b = 35 + 56
       = 91 (d)