If I was 7 when my sister was 16 and my brother was 8 when she is 19 how many more years will it take my brother to reach 21 when I'm 13?
Let the respective ages of the individual, individual's brother and individual's sister ( in years) be P, B and S.
Accordingly, P-7 = S -16; B- 8 = S - 19 , giving:
S = P + 9 = B+11, so that; B = P-2.
Substituting P =13 yields B =11.
Consequently, It will take 21 - 11 = 10 years for the brother to reach 21 years when the individual is 13 years old.