Question

How many multiples of 4 are there in {n; 37< n <1001}?

Answer 1

Let's begin by listing the first few multiples of 4:  4, 8, 12, 16, 20, 24, 28, 32, 36, 38, 40, 44.  So, between 1 and 37 there are 9 such multiples:  {4, 8, 12, 16, 20, 24, 28, 32, 36}.  Note that 4 divided into 36 is 9.

Let's experiment by modifying the given problem a bit, for the purpose of discovering any pattern that may exist:

How many multiples of 4 are there in {n; 37< n <101}?  We could list and then count them:  {40, 44, 48, 52, 56, 60, 64, 68, 72, 76, 80, 84, 88, 92, 96, 100}; there are 16 such multiples in that particular interval.  Try subtracting 40 from 100; we get 60.  Dividing 60 by 4, we get 15, which is 1 less than 16.  So it seems that if we subtract 40 from 1000 and divide the result by 4, and then add 1, we get the number of multiples of 4 between 37 and 1001:

1000
   -40
-------
 960

Dividing this by 4, we get 240.  Adding 1, we get 241.

Finally, subtract 9 from 241:  We get 232.

There are 232 multiples of 4 between 37 and 1001.

Can you think of a more straightforward method of determining this number?