Sarah's age is equal to three times Mark's age minus 4. Mark is four years older than Billy. Billy is half Ana's age. If Ana will be 15 in 3 years, how old is Sarah?
First figure out how old Ana is right now by subtracting 3 from 15: 15 - 3 = 
<gadget id="calculator">15-3</gadget>
<output>12</output>
12.
Next, figure out how old Billy is by dividing Ana's age by 2: 12 / 2 = 
<gadget id="calculator">6</gadget>
<output>6</output>
6.
Now figure out how old Mark is by adding 4 to Billy's age: 6 + 4 = 
<gadget id="calculator">6+4</gadget>
<output>10</output>
10.
Now, to figure out Sarah's age, first multiply Mark's age by 3: 10 * 3 = 
<gadget id="calculator">10*3</gadget>
<output>30</output>
30.
Finally, subtract 4 from triple Mark's age to find Sarah's age: 30 - 4 = 
<gadget id="calculator">30-4</gadget>
<output>26</output>
26.

<result>26</result>