Data:
• Total: $30
,
• Price of ribbon: 6 cents per yard
,
• Left after purchase: $27.90
Procedure:

Raina spent a total of $2.1 on ribbons.
To know how many yards she bought, we have to homogenize the units. For that, a conversion to dollars must be made.

The yard is $0.06 per yard.
Finally, we can get the yards bought:

Answer: 35 yards