The first expression is 21 garbage cans to 14 classrooms.
The ratio of the first quantity to the second is:

Hence the ratio is 3:2.
The second expression is 36 glasses of juice to 30 sandwiches.
The ratio of the first quantity to the second is:

The required ratio is 6:5.
The third expression is 12 flowers to 4 vases.
The ratio of the first quantity to the second is:

The required ratio is 3:1.
The fourth expression is 4 books to 10 pupils.
The ratio of the first quantity to the second is:

Hence, the required ratio is 2:5.
The ratios are:
2.) 3:2
3.) 6:5
4.) 3:1
5.) 2:5