Express the average of three sister in term A of the first born in years, if the second was born 5years after the first and the third was born 5 after the second.
The average age of the three sisters
F being the age of the first born;
S being the age of the seconde born and S = F + 5;
T being the age of the third born and T = S + 5 = F + 10;
The average age of the three sisters = (F + S + T)/3
And you can express the average age of the three sisters in term of the first born by substitution.
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This is what we know :
First born = a
Second born = a-5
Third born = a-5-5
Now, to calculate the average, you need to find the sum of the age of the three siblings and divide them by 3.
I hope my explanation helped you, good luck :)