Answer:
18:30:42
Explanation:
This is just like fractions, you can write 2/4 as 1/2 because 1*2=2, 2*2=4, when the numerator and the denominator of a fraction multiply with the same number (the number in my example is 2), the result will be equal to the previous fraction.
Now, let's look at this question. On the left side of = is 3:5:7, and on the right is ...:30:...
We only know that the one in the middle on the right side is 30, and the one in the middle on the left is 5, so we use 30/5=6, so 6 is the number that 5 had multiplied.
In order to get the other two blanks, all you need to do is multiply 3 and 7 with 6: 3*6=18, 7*6=42.
Now, we put the two results on their position, then the answer is 18:30:42
Hope this will help:)