<PRE><B>Question 199351</B>
A large heating unit and small heating unit are being used to heat the water
 of a pool The large unit, working alone, requires 8 hours to heat the pool.
 After both units have been working for 2 hours, the large unit is turned off.
 The small unit requires 7 more hours to complete the job of heating the water
 in the pool. 
How long would it take the small unit, working alone, to heat the pool water?
:
Let x = time required by the small unit working alone
Let the completed job (heating the pool) = 1
:
Total time by the small unit: 2 + 7 = 9 hrs
{{{2/8}}} + {{{9/x}}} = 1
Multiply equation by by 8x
8x*{{{2/8}}} + 8x{{{9/x}}} = 8x(1)
Cancel out the denominators, results:
2x + 8(9) = 8x
72 = 8x - 2x
72 = 6x
x = {{{72/6}}}
x = 12 hrs, small unit alone
:
:
Check solution:
2/8 + 9/12 = 1</PRE>