The textbook amount is calculated by multiplying full time months by $65, and part time months by $20. So the numbers in column A and/or B are multiplied accordingly, and that is the textbook credit.
http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/323/txtbk-eng.html
