Static Friction on Stacked Blocks

A block of mass m₁ = 3.0 kg is placed on top of a block of mass m₂ = 5.0 kg, which rests on a horizontal surface. The coefficient of static friction is 0.25 between the two blocks and 0.40 between the lower block and the surface. What is the maximum horizontal force that can be applied to the lower block such that the upper block does not slip?