Question 125305
Since Bill can do the job in 50 minutes then his work rate is {{{1/50}}}

Let x=no. of minutes for Ann to do it alone

Then 
Ann's work rate is {{{1/x}}}

The formula is work rate X time=work done


<table border="0">
<tr>
<td>  </td>
<td>Work rate</td>
<td><center>X time</center></td>
<td>=work done</td>
</tr>
<tr>
<td><center>Bill</center></td>
<td><center>{{{1/50}}}</center></td>
<td><center>25</center></td>
<td><center>{{{1/2}}}</center></td>
</tr>
<tr>
<td><center>Ann</center></td>
<td><center>{{{1/x}}}</center></td>
<td><center>15</center></td>
<td><center>{{{15/x}}}</center></td>
</tr>
<tr>
<td>  </td>
<td>  </td>
<td>Total work done</td>
<td><center>1</center></td>
</table>

From the table,

{{{1/2+15/x=1}}}
Multiply both sides by 2x.
{{{x+30=2x}}}
{{{30=2x-x}}}
{{{x=30}}}
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