There are several information's of immense importance already given in the question. Based on those information's the answer to the question can be easily deduced.
The total distance that has been traveled by the garden snail in question = 1/40 mile
Time taken by the garden snail to travel the distance given in question = 1/2 hour.
Then
Speed of the snail = Distance traveled/ Time taken
= (1/40)/(1/2) miles per hour
= 2/40 miles per hour
= 1/20 miles per hour
So the speed of the garden snail as can be seen from the above deduction is 1/20 miles per hour. I hope the procedure is simple enough for you to understand.
