1. Girl is 19 years
2. Mum is 38 years
3. Dad is 43 years

take girl's age=a
Mum= b
Dad= c
Then ,a+b+c=100......eq1
a=b/2.....................eq2
b=c-5
c=b+5....................eq3
sub eq 2&3 into eq1
b/2+b+b+5=100
(b+2b+2b+10)/2=100
5b+10=200
5b=190
b=190/5
b=38

sub b=38 into eq2
a=38/2
a=19

sub b=38 into eq3
c=38+5
c=43
QED



Na wa o....All these stuffs those days but till now, I've never applied them in my profession,Lol.

Let the mothers age be y
Then daughter=y/2
Then father= y+5
Mother +daughter+ father= 100
Y+y/2 + y + 5 = 100
2y + y/2 = 100 - 5
5y/2 = 95
Y = 38
Mother =38
Daughter = 38/2 = 19
Father= 38+5= 43