Hey there!
So we have two 3-number ratios, and we want them to be equivalent.
6 : __ : 15 = __ : 20 : 10
Compare 15 and 10.
15 is 1.5 times greater than 10. With that knowledge, we can change the ratio into the following.
4*1.5 : __ : 10*1.5 = __ : 20 : 10
See the pattern? The left ratio has numbers 1.5 times greater than the numbers on the right.
The first blank is the product of 20 and 1.5, which is 30.
The second blank is the first number of the left ratio divided by 1.5, which is 4.
D is your answer.