How many prime numbers exist between 260 and 280? Pick one.
A) None B) One C) Two D) Three E) Four
Straight a way lets exclude all the Even numbers between 260 and 280.   So now the number starts from 261 to 279 (Only ODD)   261 is a divisible of 3 and next odd divisible by 3 will be 261 + 6= 
<gadget id="calculator">261 + 6</gadget>
<output>267</output>
267 + 6= 
<gadget id="calculator">267 + 6</gadget>
<output>273</output>
273 + 6= 
<gadget id="calculator">273 + 6</gadget>
<output>279</output>
279.   Also we can eliminate numbers ending with '5'   So in Odd, the excluded numbers are 261,265,267,273,279, which leave us with 263,269,271,277.   Checked the above listed four numbers are divisible by any numbers till 20.   ANSWFourR:Four
<result>E</result>
How many prime numbers exist between 260 and 280?