1: Both numbers are prime numbers greater than 20: 23 and 3 1.
2: Both numbers are composite numbers less than 20: 4 and 9.
3: Two numbers, one is odd and the other is even: 4 and 5.
The greatest common denominator of 4: 42 and 30 is 6, so it can be divided into 6 groups at most, and each group gives two kinds of drinks:
Mineral water: 42/6= 7 (bottle)
Coke: 40/6= 5 (bottle)
The greatest common divisor of 5: 24, 36, 48 is 12, so these flowers can make 12 bouquets at most.
(At this time, the number of red, yellow and purple flowers in each bouquet is 2, 3 and 4 respectively. )