Multiple choice

The average age of $3$ girls is $20$ years and their ages are in proportion $3 : 5 : 7,$ the age of youngest girl is ___.

  1. $8$ years $3$ months
  2. $6$ years $8$ months
  3. $12$ years
  4. $20$ years
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Sum of ages = 3 * 20 = 60. Ratio 3:5:7 means 3x + 5x + 7x = 15x = 60, so x = 4. Youngest girl is 3x = 12 years.

AI explanation

Using the formula for average, the total age of the 3 girls is 3 times 20, yielding 60 years. If their ages are in the proportion 3 to 5 to 7, let their ages be 3x, 5x, and 7x. Adding these gives 15x equals 60, so x equals 4. The youngest girl is 3x, which is 3 times 4, giving an age of 12 years.