Answer: 9 bottles in each box and they bought 2, 4 & 5 boxes respectively.
Step-by-step explanation:
As it is clearly mentioned at the start of question that store only sells 'boxes' of juice bottles so all three of them bought perfect boxes.
So we'll find out how many boxes they'd have bought.
The largest possible divisor of 18, 36 and 45 is 9 so each box contains 9 juice bottles and guys purchased 2, 4 and 5 boxes respectively.
If you've still not understood, you can ask me for more detailed answer :)