Brandon is 3 years older than ian. the sum of their age is 27 how old is brandon ?

Brandon is 15 years old.

Let Ian's age be represented by x.
Brandon's age = x + 3

Ian's age + Brandon's age = 27
x + (x + 3) = 27
2x              = 27 - 3
2x              = 24
2x / 2         = 24 / 2
 x               = 12   * Ian's Age.

Brandon's Age = x + 3 = 12 + 3 = 15.
 
To check:

Ian's age + Brandon's age = 27
12 + 15 = 27
27 = 27