To find out the mental age of a 12-year-old child with an IQ of 90, we can use the formula:

IQ = (MA / CA) * 100

Given that the child's IQ is 90 and the chronological age (CA) is 12, we can rearrange the formula to solve for mental age (MA):

MA = (IQ / 100) * CA

Substituting the values:

MA = (90 / 100) * 12

MA = 0.9 * 12

MA = 10.8

Therefore, the mental age of the 12-year-old child with an IQ of 90 is approximately 10.8 years.