I really don't understand rate law. Could you explain that and help with this problem please?

Question

Determine the average rate of change of B from t=0 s to t=202 s.
A-->2B
Time (s) Concentration of A (M)
0 0.74
101 0.45
202 0.16

Answer 1

The tabular data is for A. The question asks for B.
From 0 sec to 202 sec the change in A was 0.74 to 0.16 or -0.58.
Therefore, rate of change for A is -0.58 M/202 sec or 0.00287 M/s.
The problem asks for B. Therefore, rate of change for B is twice that.

Answer 2

-.00574