ציטירנדיקער פּירוש אויף יהושע 1:3

בעירכם, a collective term [seeing that surely each brother had an animal of his own to ride on and to carry the grain. Ed.] Other examples of the collective term being used where a plural seemed indicated are Samuel II 10,5 עד יצמח זקנכם “until your beard (sing.) grows back.” [David had addressed several people, each of whose beard had been shaved off on one side of his face. Ed.] Joshua 1,3 is another such construction where we read כף רגלכם, “the sole of your feet,” instead of “the soles of your feet.” There are many other similar examples in Scripture.