Well...let's see....
15310 (1.2)*1 = 18372 ....nope....that's not correct !!!!
Let's use the points
(0, 15310) (1,18002) (2, 21548) ( 3, 25642) and this utility to generate a quadratic function :
http://keisan.casio.com/exec/system/14059932254941
We get the function
y = 350.5x^2 + 2402.7x + 15294.7
It appears that the last answer is pretty close to this....so...."D"