Rates P6 2010 Kong Hwa CA1 Q9

A group of painters is painting the classrooms. If they paint 6 classrooms per day, they will be able to complete 1 day ahead of schedule. If they paint 4 classrooms per day, they will be 2 days behind schedule. How many classrooms are there?


                            6 x 1 (1 day earlier)
                              |   |
[   ][   ][   ][   ][   ][   ]
[   ][   ][   ][   ]       |
                       |       |Number of classrooms
                       4 x 2 (need 2 more days)

2 u  -->  4 x 2  +  6 x 1  =  14

1 u  -->  14 / 2 = 7 days

7  -  1  =  6

6  x  6  =  36 classrooms

[or 7  +  2  =  9
      9  x  4  =  36 classrooms]

There were 36 classrooms.