Question 889110
A father can wash a car in 20 min and his son can wash a car in 5 min. If they togather start to wash a car how much time will be required?
<pre>
What??  The father takes 4 times as long as his son?  I don't believe anybody
can wash a car, and do a good job, in 5 minutes.  This problem is either
botched or very unrealistic.  However I'll do it anyway:


The father's washing rate in cars per minute is 1 car per 20 minutes or

{{{(matrix(1,2,1,car))/(matrix(1,2,20,minutes))}}}

The son's very speedy washing rate in cars per minute is 1 car per 5 minutes
or

{{{(matrix(1,2,1,car))/(matrix(1,2,5,minutes))}}}.

Suppose the answer is x minutes.

Then their combined washing rate in cars per minute is 1 car per x minutes or

{{{(matrix(1,2,1,car))/(matrix(1,2,x,minutes))}}}  

The equation comes from 

{{{(matrix(3,1,

"Father's", washing, rate))}}}{{{""+""}}}{{{(matrix(3,1,

"Son's", washing, rate))}}}  {{{""+""}}}  {{{(matrix(4,1,

Their,combined, washing, rate))}}}

{{{1/20}}}{{{""+""}}}{{{1/5}}}  {{{""=""}}}  {{{1/x}}}

Multiply through by LCD = 20x and solve.  The answer is 
x = 4 minutes.

Edwin</pre>