If Katherine has 4 apples and 3 times as many pears in her fridge, how many bananas does she have if she has a total of 21 pieces of fruit?
Katherine has a total of 3 * 4 apples = 
<gadget id="calculator">3*4</gadget>
<output>12</output>
12 pears.
She therefore had 4 apples + 12 pears = 
<gadget id="calculator">4+12</gadget>
<output>16</output>
16 apples and pears.
We know she has 21 total pieces of fruit, so this means she must have 21 total - 16 = 
<gadget id="calculator">21-16</gadget>
<output>5</output>
5 bananas.

<result>5</result>