Question 1147269
let y = the youngest sister age
:
The average age of four sisters is 24.
3s + y = 4(24)
3s + y = 96 
The average age of eldest three is 26,
3s = 3(26)
3s = 78
 how old is the youngest sister?
3s + y = 96
replace 3s with 78
78 + y = 96
y = 96 - 78
y = 18 yrs is the youngest sister