Let f be a positive function over the interval [1,4] and the area bounded by f(x) and the x-axis from x=1 to x-4 is 10. On the other hand, f is negative on the interval [4,5] and the area between f(x) and the x-axis from x-4 to x-5 is 2. Find the average value of this function on the interval [1,5].