document.write( "Question 372507: How do you solve:\r
\n" ); document.write( "\n" ); document.write( "Mark can remember only the first 4 digits of his friend's phone number. He also knows that the number has 7 digits and that the last digit is not a 0. If Mark were to dial all of the possible numbers and if it takes him 23 seconds to try each one, how long would it take to try every possibility?\r
\n" ); document.write( "\n" ); document.write( "Thank you very much! \n" ); document.write( "

Algebra.Com's Answer #265367 by Fombitz(32388) $\"\"$ $\"About$

You can put this solution on YOUR website!
So he needs the final three digits.
\n" ); document.write( "The first digit can be from 0-9.
\n" ); document.write( "The second digit can be from 0-9.
\n" ); document.write( "The third digit can be from 1-9 since he knows it's not a 0.
\n" ); document.write( "In total, then the numbers can be,
\n" ); document.write( " $\"N=10%2A10%2A9=900\"$ numbers
\n" ); document.write( "If each number takes 23 seconds, then
\n" ); document.write( " $\"T=900%2A23\"$
\n" ); document.write( " $\"highlight%28T=20700%29\"$ seconds.
\n" ); document.write( "There are 3600 seconds in 1 hour,
\n" ); document.write( " $\"T=5.75\"$ hours or 5 hours and 45 minutes.
\n" ); document.write( " \n" ); document.write( "

\n" );