[填空题]
A small box is held in place against ane it d,*/x0 wo,ie69c:r ,ajfe2bqfx rough wall by someone pushing on it3c ufkkrn2eeb*zz w o 7pt,9z9ro/,8d with a force directed upward fc oz7r/u9n,k b z2 9 pzedwt,r3o*ek8at 28$^{\circ}$ above the horizontal. The coefficients of static and kinetic friction between the box and wall are 0.40 and 0.30 , respectively. The box slides down unless the applied force has magnitude 13
N. What is the mass of the box?
