The 1:3:7 ratios mean that for each bottle of orange juice there are 3 bottles of cherry juice and 7 bottles of pineapple juice.
In particular, you can see that the number of bottles of cherry juice is three times the number of bottles of orange juice. So, if you divide the number of bottles of cherry juice by 3, you'll get the number of number of bottles of orange juice.
Then, you multiply the number of bottles of orange juice by 7 and you'll get the number of bottles of pineapple juice.