Question 677157
A large and a small heating unit are being used to heat the water of a pool.
 The large unit, working alone, requires 7 h to heat the pool.
 After both units have been operating for 3 h, the larger unit is turned off.
 The small unit requires 9 h more to heat the pool.
 How long would it take the small unit, working alone, to heat the pool
:
let s = time required by the small unit to heat the pool alone
let the completed job = 1; (a warm pool)
:
A shared work equation; the small unit was on for 3 + 9 = 12 hrs
:
{{{3/7}}} + {{{12/s}}} = 1
multiply by 7s to clear the denominators, resulting in:
3s + 7(12) = 7s
84 = 7s - 3s
84 = 4s
s = 84/4
s = 21 hrs, the small unit working alone
;
:
Check this:
3/7 + 12/21 = 
9/21 + 12/21 = 21/21 = 1