When c is increasing at a rate of 1.5×10? 2 m? s? 1 , how fast is b decreasing?

The reaction is:
A ( g ) + 1/2 B ( g ) → C ( g )
We will rewrite it so the coefficient in the front of C is 1:
1/2 A + 1/4 B → C
So the rate of increasing of C is 4 times faster than the decrease of B.
The rate of B:
- 1.5 · 10^(-2) M/s : 4 = - 0.375 · 10^(-2) M/s = - 3.75 · 10 ^(-3) M/s
  - 3.75 ·10^(-3) M/s

6/ 11 = 0.545454 so your answer is55%