In little marsden the population is 17 222. the men and women together total 9142 and the women and children together total 13 201.
there are women
there are men
there are children

M + W = 9142
W + C = 13201
Population: 17222
17222 - 13201  (W + C) = 4021
4021 = Men

M + W = 9142
so 9142 - 4021 = 5121 = Women

And W + C = 13201
so 13201 - 5121 = 8080

Check: 8080 + 5121 + 4021 = 17222
Happy to Help!

A = Children
b = men
c = women

a + b + c = 17 222
b + c = 9142
a + c = 13 201

To find a we will minus b + c from 17 222.
a + b + c = 17 222
a + (9142) = 17 222
a + 9142 (- 9142) = 17 222 (- 9142)
a = 8080

To find c we will take the equation " a + c = 13 201 " and substitute a to "a = 8080"
a + c = 13 201 
(8080) + c = 13 201
8080 (-8080) + c = 13 201 (-8080)
c = 5121

And then to find b we will take the equation " a + b + c= 17 222 " and substitute 'a' and 'c' for their solved numbers.
a + b + c = 17 222
(8080) + b + (5121) = 17 222
b + (13201) = 17 222
b + 13201 (-13 201) = 17 222 (-13 201)
b = 4021

And to check answer we will substitute all variable for their found answers.
a + b + c = 17 222
(8080) + (4021) + (5121) = 17 222

Children = 8080
Men = 4021
Women = 5121

Tada! haha 

Hope that helps. For these questions, it's a good idea to write them out as I did in the beginning to get a feel for them

 a + b + c = 17 222
b + c = 9142
a + c = 13 201

and then try to isolate and solve one variable and then go from there. :D