Let x represent the number of hours that the younger brother must work.
Two brothers decide that a 21-hour task is to be divided so that the older brother works twice as long as the younger brother. This means that the number of hours that the older brother must work is expressed as 2x.
Since the total number of hours is 21, then
x + 2x = 21
3x = 21
x = 21/3
x = 7
The number of hours that the older brother must work is