Question 930814
x = the age of the older brother
y = the age of the younger brother

The sum of the ages of the two brothers is 20.
x+y=20
  y=20-x

In five years, one is twice the other.
x+5=2(y+5)
x+5=2(20-x+5)
x+5=40-2x+10
x+5=50-2x
 3x=45
  x=15
  y=5

The older brother's age is 15 years old while the younger brother's age is 5 years old.