There are 15 rows of diamond rings in a case in the jewelry store. there are 24 rings in each row. another case has 12 rows of rings with a variety of colored stones and has 18 rings in each row.
how many more diamond rings than rings with colored stones are in the jewelry store?

In jewelry store we have 15 rows and each has 24 rings so in total we have :
15 * 24 = 360 rings and in another case we have 12 rows and each has 18 rings so in total we have : 12 * 18 = 216 so jewelry store has (360 - 216) rings more than other case so jewelry store has 144 rings more than other case :)))
i hope this is helpful 
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15 rows* (24 diamond rings/ 1 row)= 360 diamond rings.

12 rows* (18 rings with colored stones/ 1 row)= 216 rings with colored stones.

Note that the units cancel out so you get the answer.

360 rings- 216 rings= 144 rings.

There are 144 more diamond rings than rings with colored stones in the jewelry store~

The answer is b.$19.99  hope this : )