.
For such simple problem, than simpler the solution is, than better you understand it.
If the average age of four sisters is 24 years, then the sum of their ages is 4*24 = 96 years.
If the average age of the three oldest sisters is 26, then the sum of their ages is 3*26 = 78 years.
If the age of the youngest sister is x, then you have this simple equation
x + 78 = 96,
from which you get the ANSWER momentarily
x = 96 - 78 = 18.
Solved.
When you learn on how to solve such problems, you will be able do it in one line, as THIS :
ANSWER = 4*24 - 3*26 . . .
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To get more practice in solving problems on average values, look into the lessons
- Solved problems on average scores, weight, height and temperature
- Solved problems on average scores
- Solved problems on average age
in this site.
Consider these lessons as your textbook, handbook, tutorials and (free of charge) home teacher.