Hey there! :D
Let's keep the fraction 5/7 in mind. We should be able to reduce everything to 5/7.
15:21 (divide both by 3) you get 5:7. That is equivalent.
10 to 14. Divide both by 2. 5 to 7. That is equivalent.
The correct answer is 28/20. That is not equivalent.
I hope this helps!
~kaikers