tolong banget yaa nomer 13-15 beserta caranya yaaa buat materi UAS bsk pleasee

13. 79 x 11 = 869
      869 - 64 = 805
      805 / 10 = 80,5 ( Rata-rata )

14. 80 x 28 = 2240
      ( 2240 + x + 3x ) / 30 = 78
      ( 2240 + 4x ) /30 = 78
       2240 + 4x = 2340
       4x = 100
        x = 25
       ( 3 x 25 ) - 25 = ...
       75 - 25 = 50

15. 75/100 x 40 = 30 siswa.

Semoga ngerti yah.... emang agak ribet sih hehe ^v^