you have to put  3 gutters of different sizes together  and make lengths. the problem is to find out how many  such lengths can be made. 

the minimum length that can be made is 18
 ( using three 6 feet units ).
maximum length is 30
 ( using 10 feet blocks )
all the even number lengths in between can also be made
 ( as the difference in size of the blocks is 2 )
which means all the even number lengths between 18 and 30
( both inclusive ) can be made .

the combinations are given below  for u to understand better

666         18
668         20
6610       22
6810       24
61010     26
81010     28
101010   30

so the answer is 7

note
there should be a  better way to solve this problem using  permutaion/ombination principles