I’ve always been terrible at mental arithmetic. At primary school, we had to listen to tapes of the times tables set to music, then do a test. You started with the two times table, and if you passed, the next week you did the three times table. It didn’t really work me, I was stuck on the three times table for aaages. At high school, I remember my maths teacher being amazed that I’d got 96% in algebra, 100% in shape and space, 87% in statistics… and 17% in number.
It’s a bit of a flawed policy to hold students back because of a weakness in one area, especially if they excel in others. I wish I was better at mental arithmetic, but I can’t say that my lack of ability in that area (or interest, to be honest) has ever held me back in life.