Question 1010639
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Out of 40 children some pay Rs 5 each while the rest contribute Rs 10 for an excursion. If the total collection is Rs 320 how many gave Rs 5?
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One can solve the problem by reducing it to a system of two equations or to a single equation.


More elegant way is to use the common sense and mental math.


Let us assume for a minute that everybody contributed  Rs 10. 

Then the total collection would be  40*10 = Rs 400,  which is in  RS 80 more than  Rs 320.

It is clear that the difference arose because some children payed   Rs 5  that we intently counted as  Rs 10. 

It is also clear that the number of those who payed  Rs 5  is  {{{80/(10-5)}}} = {{{80/5}}} = 16.


It gives the answer:  16 children payed  Rs 5  and  40-16 = 24  payed  Rs 10.


For more problems similar to this one see the lesson <A HREF=http://www.algebra.com/algebra/homework/word/misc/What-type-of-problems-are-these.lesson>What type of problems are these?</A> in this site.