E. Chou, 9 years old, finishes Drills 1 and 2 with almost no mistakes in 1'07".
A 9-year-old IQ Abacus student was taking the Certification Exam conducted by The US Mental Math Foundation.
He was using the Abacus Mental Math method in solving the 4 to 5 digits +/- questions. At the end of the 3-minute test, he finished 12 questions, of which 11 were correct. Can you detect the question that he missed?