M(n) is the array in which m(x,y,z,t,u)=x*y*z*t*u for x = 1 to n, y = 1 to n, z = 1 to n, t = 1 to n and u = 1 to n. In M(7), the most frequently occurring numbers are 360, 420, 504 and 840, each occurring 300 times. The smallest of these numbers is 360, so a(7) = 360.