You see the boy next door trying to push a crate down the sidewalk. He can barely keep it moving, and his feet occasionally slip. You start to wonder how heavy the crate is. You call to ask the boy his mass, and he replies " 49.0kg ." From your recent physics class you estimate that the static and kinetic coefficients of friction are 0.900 and 0.4 for the boy's shoes, and 0.5 and 0.2 for the crate.

Estimate the mass of the crate.