Question 923126
Hank can wash and wax the car in 2 hours
so in 1 hr , he can do   1/2 of the work
Sally can do it in 1 hour, 30 minutes (1.e 1.5 hr)
so in 1 hr he can do   1/1.5 of the work
three persons together can do it in 40 min  ( 40 min  = 40/60  = 2/3 hr)
three persons together can complete in 2/3 hr
In 1 hr they can do {{{1/(2/3)}}} work
                     = {{{3/2}}} work
I hr, lena alone can do =  {{{ 3/2- 1/1.5 - 1/2}}} of the work  
                        ={{{3/2 -1/(3/2) -1/2}}} of the work
                         ={{{ 3/2- 2/3-1/2}}} of the work
                 multiply and divide with 6 to each term
                     = {{{(3/2) *(6/6)- (2/3)*(6/6)-(1/2)*(6/6)}}} of the work
                      ={{{(3/1) *(3/6)- (2/1)*(2/6)-(1/1)*(3/6)}}} of the work
                      ={{{9/6- 4/6-3/6}}} of the work
                        = {{{ (9-4-3)/6}}} of the work
                        = {{{1/3}}} of the work
so , 1 Hr Lena can do 1/3 of the work
so to complete full work , lena can take 3 hrs
so result is 3 hrs