It takes David 10 minutes to wash 4 windows.  David's house has 64 windows.  How many minutes will it take David to wash all of the windows?
It takes 10 minutes to wash 4 windows and he has 64 windows so that breaks down to 64/4 = 16 units
It takes 10 minutes to wash a unit of windows and he has 16 units so 10*16 = <<10*16=160>>160 minutes
#### 160