Suppose the coal consumption per day is X, then the coal yard * * * stores 45x coal.

The daily coal storage capacity of large trucks is (45x+5x)/ 10=5x.

The daily coal storage capacity of small trucks is (45x+9x)/(4*9)=27x/ 18.

So it takes y days for two big trucks and four small trucks to pull together. The equation is as follows

(2*5x+4*27x/ 18)y=y*x+45x

( 10+54/9)y=45+y

15y=45

y=3

Therefore, it takes 3 days to fill the coal yard!