Question

5. The average age of 3 sisters is 15. If the ages of 2
sisters are 12 years and 15 years, the age of the third
sister is-​

Answer 1

Answer:

let the age if the unknown girl be x

so average of all sister is equal to,

12+15+x/3=15

27+x=3*15

27+x=45

x=45-27

x=18

so the third sister's age will be 18 years

Answer 2

Question:-

The average age of 3 sisters is 15. If the ages of 2 sisters are 12 years and 15 years, the age of the third sister is .... ?

Solution:-

{ According to question}

• The average age of 3 sisters is 15
• The ages of 2 sisters are 12 years and 15 years.

Let, age of third sister be x

Now we use average formula

${ \bf{ \dashrightarrow{ \purple { \frac{Sum \: of \: age \: of \: 3 \: sister's}{Number \: of \: sister} = Average \: age}}}}$

${ \bf{ \dashrightarrow{ \purple { \frac{12 \: + \: 15 \: + \: x}{3} = 15}}}}$

${ \bf{ \dashrightarrow{ \purple { {12 \: + \: 15 \: + \: x} = 15 \times 3}}}}$

${ \bf{ \dashrightarrow{ \purple { {27+ \: x} = 45}}}}$

${ \bf{ \dashrightarrow{ \purple { {x} = 45 - 27}}}}$

${ \bf{ \dashrightarrow{ \purple { {x} = 18}}}}$

Hence age of third sister is 18 year's.

i hope it helps you.