Do these ratios form a proportion?
20 crates : 15 seconds
28 crates : 21 seconds
Yes, these ratios form a proportion.
To check, we can find the cross products of the ratios:
20 crates * 21 seconds = 28 crates * 15 seconds
420 crates*seconds = 420 crates*seconds
The cross products are equal, which means the ratios form a proportion.