How many 4 digit numbers have odd number of factors?

1.	How many 4 digit numbers have odd number of factors?
Answer» $1000, 1001, \cdot\cdot\cdot,9998,9999\:are\\Four \:digit \: numbers$ $\pink { Every\: perfect \: square \:has \:<klux>ODD</klux>}$ $\pink { number \:of \: Factors. }$ $We \:know \:that ,$ 31² = 961 ( 3 digit number ) 32² = 1024 ( 4 digit FIRST number which has odd number of factors) 99² = 9801 ( 4 digit last number which has odd number of factors ) $Number \:of \:4 \:digit \: numbers \: have \:odd\\number \: of \:factors\\ = 99 - 32 + 1 \\= 99 - 31 \\= 68$ •••♪

Answer»

$1000, 1001, \cdot\cdot\cdot,9998,9999\:are\\Four \:digit \: numbers$