The lake is 20 miles across because 2 x 10 = <<2*10=20>>20
If takes .5 hours to go from bottom to top because 30 / 60 = <<30/60=.5>>.5
The lake is five miles from bottom to top because 10 x .5 = <<10*.5=5>>5
The square mileage is 100 because 20 x 5 = <<20*5=100>>100
#### 100