Question 284774
We can work this from the view of the herbicide or from the view of the water.
.10h=5*.04
.10h=.20
h=2
start with 2 gallons of herbicide and add 3 gallons of water.
THe question "How much of the 10% solution did the landscaper have before he added the water?" is poorly worded.
We don't know maybe he had just 2 gallons maybe he had a warehouse of it. 
But he only used 2 gallons of the 10% herbicide.

from the view of the water
.90h+(5-h)=.96*5
.9h+5-h=4.8

.9h-h=-.2
-.1h=-.2
h=2 gallons of herbicide
water=5-2=3 so 3 gallons of water