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Work Done by resistance force and applied force

In my book there is a question that is, A box is pushed throgh 4 m on a horizontal surface and surface offering resistance of 100 N. Find the work done by (1)applied force (2) resisting force?

now, solution is also given in the book, both answer is same but only difference of sign.One is positive and one is negative. solution  are given as follow-:

(1)work done by applied force=f×s×cos(theta)
= 100×4×cos 0 degree
=400joule

(2)work done by resisting force=f×s×cos (theta)
=100×4×cos180 degree
=  -400joule

I understand the second answer but i dont understand the 1st answer how work done by applied force is 400 j,since how in case of 1st force is 100 N. The resisting force is 100N, not applied force how in the 1st case applied force is 100N

simply my question is  – in the case of 1st how force is taken as 100N i think here we should take value of applied force and we are taking the value of resisting force.

For enabling motion the applied force must at least be equal to the resistance force in opposite direction.