;Program to teach Subtraction
;By: Rohit Kumar
;Dt: 12/10/02

(defun teach-subtraction ()
	(print "Hello. So i ll be teaching you to subtract numbers now.")
	(print "Press ENTER or RETURN key to move ahead in this tutorial")
	(get-key)
	(print "Let us start by subtracting 4 from 7. 7 - 4 = ?")
	(get-key)
	(print "Start counting from 6 in decreasing order on your fingers.");
	(print "6, 5, ...")
	(get-key)
	(print "Do this for 4 numbers.")
	(get-key)
	(print "So what do we have 6, 5, 4, 3.");
	(print "Ok we have 3 (Three). It is the answer.")
	(get-key)
	(print "See here we started counting from 6 because 6 is one less than 7.")
	(print "")
	(get-key)
	(print "Note that when doing b - a, b must be more that a.")
	(get-key)
	(print "So if b is less than a then do a - b and put a minus in front of the answer.")
	(get-key)
	(print "For example to do 4 - 7, you must do 7 - 4 as above.")
	(print "Then put - in front of the answer ie -3. This is the answer")
	(get-key)
	(print "OK. Now we move to subtracting numbers with 2 or more digits.")
	(get-key)
	(print "Lets do : 37 - 12")
	(print "  37")
	(print "- 12")
	(print "----")
	(get-key)
	(print "This is similiar to single digit subtraction.")
	(print "Start doing a column single digit subtraction as above.")
	(print "Start from the right side and write the answer under that very column.")
	(get-key)
	(print "Lets try.")
	(get-key)
	(print "First column 7 - 2 = ?");
	(get-key)
	(print "Count on your fingers 6, 5. OK 5 (five).")
	(get-key)
	(print "Write it under right most column.")
	(get-key)
	(print "Here it is...")	
	(print "  37")
	(print "- 12")
	(print "----")
	(print "...5")
	(print "----")
	(get-key)
	(print "Similiarly do for the next column.")
	(get-key)
	(print "We have 3 - 1 = ?")
	(get-key)
	(print "So that is 2 (two). Write it down")
	(get-key)
	(print "What do we have now ?")
	(print "  37")
	(print "- 12")
	(print "----")
	(print "  25")
	(print "----")
	(get-key)
	(print "So this is done")
	(get-key)
	(print "Again had it been 12 - 37 = ? :")
	(get-key)
	(print "We would have done 37 - 12 and added - to the result")
	(print "So the answer would have been -25")
	(get-key)
	(print "Now lets try 32 - 17 = ?")
	(print "  32")
	(print "- 17")
	(print "----")
	(get-key)
	(print "First column 2 - 7 = ?");
	(get-key)
	(print "Now here we cannot do 7 - 2 and put a minus.")
	(get-key)
	(print "Instead we borrow a one from 3 and put it in front of 2.")
	(get-key)
	(print "So now 3 becomes 2 and 2 becomes 12")
	(print "See below")
	(print "  2 12")
	(print "- 1  7")
	(print "------")
	(get-key)
	(print "Now try doing it.")
	(get-key)
	(print "First column from right.")
	(get-key)
	(print "Count on your fingers 11, 10, 9, 8, 7, 6, 5. OK 5 (five).")
	(get-key)
	(print "Write it under right most column.")
	(get-key)
	(print "Here it is...")	
	(print "  2 12")
	(print "- 1  7")
	(print "------")
	(print ".....5")
	(print "------")
	(get-key)
	(print "Similiarly do for the next column.")
	(get-key)
	(print "We have 2 - 1 = ?")
	(get-key)
	(print "So that is 1 (One). Write it down")
	(get-key)
	(print "What do we have now ?")
	(print "  2 12")
	(print "- 1  7")
	(print "------")
	(print "  1  5")
	(print "------")
	(get-key)
	(print "So this is done. Answer is 15 (fifteen)")
	(get-key)
	(print "OK last question now. 3 Digits.")
	(print "Lets try 256 - 352 = ?")
	(get-key)
	(print "Now since 352 is more than 256 we will first do 352 - 256 = ?")
	(get-key)
	(print "  352")
	(print "- 256")
	(print "-----")
	(get-key)
	(print "First column 2 - 6 = ?")
	(get-key)
	(print "So we need to borrow 1.")
	(print "This becomes ... ")
	(get-key)
	(print "  34 12")
	(print "- 25  6")
	(print "-------")
	(get-key)
	(print "First column from right.")
	(get-key)
	(print "Count on your fingers 11, 10, 9, 8, 7, 6. OK 6 (six).")
	(get-key)
	(print "Write it under right most column.")
	(get-key)
	(print "  34 12")
	(print "- 25  6")
	(print "-------")
	(print "......6")
	(print "-------")
	(get-key)
	(print "Second column 4 - 5 = ?")
	(get-key)
	(print "Again we need to borrow 1.")
	(print "This becomes ... ")
	(get-key)
	(print "  2 14 12")
	(print "- 2  5  6")
	(print "---------")
	(get-key)
	(print "Second column from right.")
	(get-key)
	(print "Count on your fingers 13, 12, 11, 10, 9. OK 9 (nine).")
	(get-key)
	(print "Write it under right most column.")
	(get-key)
	(print "  2 14 12")
	(print "- 2  5  6")
	(print "---------")
	(print ".....9  6")
	(print "---------")
	(get-key)
	(print "Finally third column 2 - 2 = ?")
	(get-key)
	(print "This is 1, 0. Ok 0 (zero).")
	(get-key)
	(print "Finally it becomes...")
	(print "  2 14 12")
	(print "- 2  5  6")
	(print "---------")
	(print "  0  9  6")
	(print "---------")
	(get-key)
	(print "So 352 - 256 =  96.")
	(get-key)
	(print "So 256 - 352 = -96.")
	(get-key)
	(print "So you have learnt to subtract numbers now.")
	(get-key)
	(print "You can try to subtract 1024 - 512 = ?  and 302 - 278 = ?")
	(get-key)
	(print "OK Good Bye.")
)