A train travels between 3 different cities.  It goes 75 miles from the first city to the second city, 100 miles from the second city to the third city, and 50 miles less than that combined distance to go from the third city to the first city.  It does this trip 3 times a day.  The train needs service every 18,000 miles so how many days does it go between services?
It takes 100+75=
<gadget id="calculator">100+75</gadget>
<output>175</output>
175 miles for the first two legs of the trip.
The third leg is 175-50=
<gadget id="calculator">175-50</gadget>
<output>125</output>
125 miles.
So the total trip is 175+125=
<gadget id="calculator">175+125</gadget>
<output>300</output>
300 miles.
So it goes 3*300=
<gadget id="calculator">3*300</gadget>
<output>900</output>
900 miles per day.
So it can go 18000/900=
<gadget id="calculator">18000/900</gadget>
<output>20</output>
20 days between services.

<result>20</result>