She earned $10.00.

(Working backward)

Step 1: Begin with the amount they had left: $2.74.

Step 2: Find out how much they spent.

Step 3: Add the amount spent to the amount they had left.

Step 4: Subtract the cost of the exercise wheel they returned.

$20.69
  – 5.69
$15.00 amount brought to the store

Step 5: Subtract the amount of money Lee contributed.

$15.00
  – 5.00
$10.00

Nicki had earned $10 last week.