We must find UNIQUE combinations because choosing a,b,c,d... is the same as d,c,b,a...etc. For this type of problem you use the "n choose k" formula...
n!/(k!(n-k)!), n=total number of choices available, k=number of choices made..
In this case:
20!/(10!(20-10)!)
20!/(10!*10!)
184756
→ 
the ratio of PR to QS is 1:1 because they are congruent (equal)
If you draw a picture, you will see that PQRS is a parallelogram.
14.5% = 0.145
Step-by-step explanation:
The term 'percent' (%) actually means per 100. So when you say 14.5% it means 14.5 per 100. So when you put that into an equation it actually means:
So when you solve it, 14.5% is actually 0.145 and not 14.5.
Your friend is then incorrect because:
14.5% ≠ 14.5