- There are eight Prime Numbers Under 20
- Many people are unsure whether they have listed them all so it can help to draw the grid out and put them all into it – 4 under 10 then 4 more between 10 and 20
- One is not a prime
- Two IS a prime

I invented “Cross Sums” to help students who struggle with Proportional Reasoning. The philosophy includes:
These work best if they are printed onto card, laminated, then cut up. To play, shuffle them, place them face up, and pick an easy question. Find the answer to that question, chain that card on, repeat…
I would recommend Mel’s post on error intervals – her definition of an Error Interval is:
“The range of values (between the upper and lower bounds) in which the precise value could be”
and she gives a clear explanation of how to find one.
Here is an easy worksheet http://www.mathsmadeeasy.myzen.co.uk/gcse-maths-revision-papers/grade-c/60_algebra_changing-the-subject.pdf
And this one goes to a much higher level http://www.mash.dept.shef.ac.uk/Resources/A8chsubj.pdf
Mathwarehouse.com is *very* buggy website but oday one of my students did the parallel line problems and thoroughly enjoyed them.