What is the rate of 2.50/10.50 and 1.99/2.50

Unsure of what you're asking by "What is the rate of 2.50/10.50
and 1.99/2.50?"  Do you mean, "What is the RATIO of 2.50/10.50
and 1.99/2.50?"

If so, this ratio is 

2.5/10.5

1.99/2.50

Invert the denominator, obtaining 2.50/1.99, and then multiply the numerator 2.5/10.5 by this result:

2.5        2.5        6.25
* =  =  1 / 3.3432, or 0.2991.
10.5      1.99      20.895

If this is not the question you intended to share, please fix up your original post so I'll know what you meant.

She should use 12/3 to measure

Dis the correct answer