Answer:
4 packs of party plates and one pack of straws.
Explanation:
We have been given that there are 30 plates in a pack and 120 straws in a pack she needs exactly the same number of plates as straws.
To solve our given problem, we will convert find least common multiple of 30 and 120.
Multiples of 30: 30, 60, 90, 120,150,
Multiples of 120: 120, 240,...
We can see that least common multiple of 30 and 120 is 120.


Therefore, Helen should buy 4 packs of party plates and one pack of straws.