Ok... i have these calculations i did in math class.
My estimates are there!
But i'll write them out:
Book one: pick; 15, heather 17... book two: pick, 16-17ish, heather, 18-19ish...
Book three: pick; 17-18ish, heather, 20.
Book 4: pick, 18-20... heather, 21 or 22..
But my calculations still seemed off for those beside book one....