Fundamentals
Applying abstraction skill
Knowing the theoretical basis
John bought a dozen apples and received 4 more from his father. John gave 2 apples to his sister, 3 to his brother, and among the rest, 5 were spoilt and were dumped. How many apples left did John have?
Answer: 12 + 4 - 2 - 3 - 5 = 6