Ali is twice as old as his youngest brother. If the difference between their ages in 15 years, how old is his youngest brother?

Ali is twice old as his youngest brothr
Age difference never changes, after 15 years, both will increase by 15, so the difference will remain the same.
If Youngest brother is X
Then, Ali is 2x
2x – x= x
Given Difference= 15
X=15