Question 884374
What fraction of the cars did Stefan wash in 2.5 hrs?
His rate of washing is:
( 1 job ) / ( 6 hrs ) = {{{ 1/6 }}}
Note that I call washing all the cars 1 job
------------------------------------
Let {{{ x }}} = fraction of cars he washes in 2.5 hrs
{{{ 1/6 =  x/2.5 }}}
Multiply both sides by {{{ 6*2.5 }}}
{{{ 2.5 = 6x }}}
{{{ x = 2.5/6 }}}
{{{ x = 5/12 }}}
--------------
So, Stefan washes 5/12 of the cars in 2.5 hrs. That
means there are 7/12 of the cars left to wash
-----------------------------------------
Misha takes over by himself. His rate is:
( 1 job ) / ( 5 hrs ) = {{{ 1/5 }}}
Let {{{ t }}} = time in hrs for him to finish washing cars
by himself
{{{ 1/5 = ( 7/12 ) / t }}}
{{{ 1/5 = 7/( 12*t ) }}}
Multiply both sides by {{{ 5*12*t }}}
{{{ 12t = 5*7 }}}
{{{ t = 35/12 }}}
{{{ t = 2 + 11/12 }}}
------------------
How many minutes is {{{ 11/12 }}} hrs?
{{{ (11/12)*60 = 55 }}}
---------------------
Misha takes 2 hrs and 55 min to finish washing the cars