Respuesta :

Answer:

10

Step-by-step explanation:

Because every number goes back the number increases by 1.

31 to 30 ( - 1 )

30 to 28 ( -2)

28 to 25 ( -3)

25 to 21 ( -4 )

21 to 16 ( -5 )

16 to ? ( -6)

? = 10