tamarind, this is a form of 'number patterns' questions.tamarind wrote: Are maths puzzles supposed to be like this ?

the answer can be derived from 50+30+10=90; 20+40+ ? =90; ? = 90-40-20 = 30.

number bond method of 50-39 ischamonix wrote:

He only has to write down how he derived it, i.e 51-39 = 50-40+2. Your son is great in Math. Was he attending preschool in US?

The school textbook does teach P1 kids to add/subtract in a way (regrouping) similar to your ds' method, i.e 50 =50+1 but not 39=40-1.

Mine faces the same problem of not showing workings. But, I suppose this issue would be quite common, especially for those kids who do abacus or other Math enrichments...

50* - 39** = (40-30)+(10-9) = 10+1 = 11

*40+10 **30+9

If the syllabus teaches numberbonds subtraction, i think some teacher may not give full marks for using other methods of working like in abacus method regardless if the answer is correct. They may even deduct half a mark if they are so particular.

I think school teachers will deduct marks for not showing workings. I even heard of them deducting marks for not labelling the workings properly (ie not writing down the key words).