For a group of 12 years old children the chronological age $CA=12 $ years

The given group of 12 years old children has $80 \leq IQ \leq 140$

Substituting $IQ = \large\frac{MA}{12} $$\times 100 \leq 140$----(1)

Multiplying by positive number $\large\frac{12}{100}$ through out the inequality we get,

=> $80 \times \large\frac{12}{100} \leq \large\frac{MA}{12} $$ \times 100 \times \large\frac{12}{100}$$ \leq 140 \times \large\frac{12}{100}$

=> $9.6 \leq MA \leq 16.8$

Thus we see that the range of metal age of 12 years old children is more than or equal to 9.6 and less than or equal to 16.8

The solution set is $9.6 \leq MA \leq 16.8$