Bread and sugar cost 110 together, bread cost 100 more than sugar, how much does sugar cost?

First write an equation system based on the problem
We can write "Bread and sugar cost 110 together" as
∴ b + s = 110
We can write "bread cost 100 more than sugar" as
∴ b = 100 + s

Second, solve the equation system by subtitution method
Subtitute b with (100+s) in the first equation, and we'll find the value of s
b + s = 110
(100 + s) + s = 110
100 + 2s = 110
2s = 110 - 100
2s = 10
  s = 10/2
  s = 5
The cost of the sugar is 5