Then Meng needs to go back to concrete material, working through the subtraction step by step decomposing numbers made with Multi-Base Arithmetic Blocks. As all the questions needed regrouping, Pepu failed to get any of them correct. Whenever a larger number has to be taken away from a smaller number, Pepu puts down the result as 0. Pepu might be helped by thinking about how the algorithm leads to some silly answers on some easy questions. It might also be worth gently probing to see if Pepu has any idea of negative numbers e.g. in a temperature context.
Some children automatically invent a strategy for subtraction based on negative numbers.