Answer:
Ken is in Piaget's concrete operational stage.
Step-by-step explanation:
According to Piaget, the concrete operational stage begins when children are around 7 years old. In this stage, logical thought begins to develop and children become more logical, nonetheless, their thinking is still very concrete. Their problem solving abilities are still narrowed to concrete and specific things, like Ken, who is able to perform mathematical operations if he can use real objects or familiar examples.
The concrete operational stage is followed by the formal operational stage.