Keith uses 18 cherries and 3 peaches to make a pie filling. Lena uses an equivalent ratio of cherries to peaches when she makes pie filling. Can Lena use a ratio of 21 cherries to 6 peaches?

No, she cannot use a ratio of 21 to 6. You can figure this out in a few different ways. First you can simplify down the fraction 18/3, to get 6/1. 6/1 does not equal 21/6, because 21/6 equals 7/2. However, 6/1 it does equal 36/6. You could also noticed that three doubles to make six, yet 18 does not double to make 21