A very simple approximate analytic solution, accurate to the order of rV/K, is given for the interplanetary propagation of solar cosmic rays. The solution includes effects of convection, diffusion, and adiabatic deceleration. Comparision with previous numerical and Monte Carlo calculations shows that the solution is accurate for K>5¿1020 cm2/s. The model gives an exponential temporal decay for late times in the long-lived events. Fits to the November 15, 1960, and November 22, 1977, ground level events are given.