Question

What number should be substituted for the question mark to balance the equation and why is that the correct number?

Answer 1

@halorazer

Answer 2

You could make an equation out of it.
8(O2)+10O
Where you'd distribute the 8 to the 2 and get 16
16O+10O=?O2
26O=?O2
Then divide by 2
26/2=13
B.

Answer 3

Thank you!