eight-year-old harvey has a mental age of four. what is his intelligence quotient?

Computation of intelligence quotient is mental age/chronological age (real age)*100.

Harvey is 8 years old (Chronological age)
                 4 years old (mental age)

               =(4/8)*100
               = 0.5*100
               =50  intelligence quotient 

Based on Binet's scale 70% and below belongs to the cluster of below normal IQ levels and Harvey is included in this cluster.