<prompt> IQ is calculated by -Mental agechronological ageMental age - chronological ageChronological agemental ageChronological age - mental age  <response>: Ans. is a i.e., Mental ageChronological age The Intelligence Quotient I.Q. is calculated according to the formula:The WHO classification of mental retardation is based on the concept of IQ and includes the following categories:CategoryIQMild50-70Moderate35-49Severe20-34Profound20A child with IQ 70 is called mentally retarded. 