<PRE><B>Question 180966</B>
Let x=amount of time it takes Chris working alone to clean the house
Chris therefore works at the rate of 1/x house per hour
Mary and Chris working together works at the rate of 1/2 house per hour
Mary alone works at the rate of 1/(3.5) house per hour

Now we are told that:
 1/x + 1/(3.5)=1/2 multiply each term by 7x
7+2x=3.5x  subtract 2x from each side
7=3.5x-2x
1.5x=7
x=4 2/3 hours------------------amount of time it takes Chris working alone
4 2/3=14/3 
3 1/2=7/2
3/14 +2/7=1/2
3/14+4/14=1/2
1/2=1/2

Hope this helps---ptaylor</PRE>