Answer:
18:21 is your answer.
Explanation:
When dividing 24/28 you get:
24 : 28 = 6 : 7
Now lets look at the 1:4 . 4:7 , 12:16 , 18:21
1 : 4 does not equal 6 : 7.
4 : 7 does not equal 6 : 7.
12 : 16 does not equal 6 : 7.
18 : 21 does equal 6 : 7.
18:21 is your answer.