Anders and Petra and Karin are together 18 years old.Anders is half the age of Karin.Petra is half the age of Anders and Karin's age togethe

                                                                    A + P + K = 18      ...(1)

A = K/2          Multiply both sides by 2 to get  2A = K                   ...(2)

                                                                    P = (A + K)/2        ...(3)

 

Replace K in (1) and (3) using (2)

A + P + 2A = 18      or    3A + P = 18             ...(4)

P = (A + 2A)/2        or    P = 3A/2                  ...(5)

 

Replace P in (4) using (5)

3A + 3A/2 = 18      or 9A/2 = 18   so  9A = 36  or A = 4

 

Use A in (5) to get P = 3*4/2 or P = 6

 

Use A in (2) to get K = 2*4 or K = 8

 

Anders is 4, Petra is 6, Karin is 8