The pirates plan to explore 4 islands. Two islands require walking 20 miles per day while the other two islands require 25 miles per day. How many miles will they have to walk if it takes 1.5 days to explore each island?
It will take 1.5 days x 2 = <<1.5*2=3>>3 days to explore 2 islands.
So, the pirates will walk 20 miles x 3 days = <<20*3=60>>60 miles to explore two islands.
The other 2 islands will be explored by walking 25 miles x 3 days = <<25*3=75>>75 miles.
Thus, the pirates will need to walk 60 + 75 = <<60+75=135>>135 miles to explore the 4 islands.
#### 135