Maria's test scores are 80, 70, and 90. What score does she need to get on a fourth test so that her average score for the four tests is exactly 85?
She needs to have a total of 4*85=
<gadget id="calculator">4*85</gadget>
<output>340</output>
340 points over the four tests.
She has 80+70+90=
<gadget id="calculator">80+70+90</gadget>
<output>240</output>
240 points so far.
She needs 340-240=
<gadget id="calculator">340-240</gadget>
<output>100</output>
100 on the fourth test.

<result>100</result>