A thread about maths should have some statistics 
Percentage of candidates gaining grade %gaining Number
grades graded
A* A B C D E N U A - E
Maths
2018 15.9 26.3 22.6 16.0 10.0. 5.5 3.7 96.3 97627
Maths Further
2018 28.8 29.0 19.2 11.0 6.3 3.3 2.4 97.6 16157
From http://www.bstubbs.co.uk/a-lev.htm
And only 16.5% of those doing maths are also doing further maths.
~ 42% of single maths students get Astars or As - 41,198
If we assume that the students who do FM and her Astar, A or B will get Astars and As in the single (this assumption isn't necessarily valid but seems like a fair guess) that's 12,440. Which leaves 28,757 single maths students getting Astars and As which is 29.5%
If you look at the link, other subjects don't have so many A star and A grades awarded.
E.g physics, which is probably the most directly comparable subject.
2018 9.4 20.2 20.7 19.8 16.0 9.7 4.2 95.8 37806
...29.6% A stars and As.
You might want to get your kids to check my sums, it's been a long day...
The numbers of top grades are, it appears, weighted to account for the fact that FM is largely a self-selecting group of exceptionally bright students who also of necessity do single maths.