Modeling the behavior of granular media. Granular media are substances made up of many distinct grains—including sand, rice, ball bearings, and flour. The properties of these materials were theoretically modeled in Engineering Computations: International Journal for Computer-Aided Engineering and Software (Vol. 30, No. 2, 2013). The model assumes there is a system of N non-interacting granular particles. The particles are grouped according to energy level. Assume there are r energy levels, with Ni particles at energy level i, i = 1, 2, 3,c, r. Consequently, N = N1 + N2 + g+ Nr. A microset is defined as a possible grouping of the particles among the energy levels. For example, suppose N = 7 and r = 3. Then one possible microset is N1 = 1, N2 = 2, and N3 = 4. That is, there is one particle at energy level 1, two particles at energy level 2, and four particles at energy level 3. Determine the number of different microsets possible when N = 7 and r = 3.