Question 1198001
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A man bought 15 books for #9800. 
Some of these books cost #600 each and the rest cost #700 each. 
How many of each type did he buy.
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If all 15 books cost #600 each, the total would be 15*600 = 9000.


Comparing it with #9800, we conclude that 8 books cost #700 each, 
while the rest 15-8 = 7 books cost #600 each.


<U>ANSWER</U>.  7 books at #600 each and 8 books at 700 each.


<U>CHECK</U>.  7*600 + 8*700 = 4200 + 5600 = 9800, which is precisely correct.
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Solved mentally, without using equation/equations, by mental reasoning, only.