The correct verb form is "joins".
The sentence refers to a general or habitual action, which requires the present tense. Since the subject "brook" is singular, we use the present tense form "joins".
The sentence becomes:
The brook joins the river here.
The correct verb form is "joins".
Arun’s present age in years is 40% of Barun’s. In another few years, Arun’s age will be half of Barun’s. By what percentage will Barun’s age increase during this period?