Answer: Combinations will be 
1:3, 3:9, 2:6 ..........
Explanation:
Since we have given that 
Trudy is making punch by mixing 1 can of concentrated juice with 3 cans of water 
So, Number of can of concentrated juice = 1
Number of cans of water = 3
so, the ratio of juice to water is given by
Juice : Water
 1 : 3
So, combinations will be 
1:3, 3:9, 2:6 ..........