A flask is charged with 0.124 mol of A and allowed to react to form B according to the reaction A(g) →B(g). The following data are

Question

obtained for [ A ] as the reaction proceeds:
Time (s) 1 10 20 30 40
Moles of A 0.124 0.110 0.088 0.073 0.054

how do i solve for B?
for problem 8: also i know how to solve for A but i cant figure out the one below, i tried subtracting
0.088 - 0.073/20 = but its wrong

7) The average rate of appearance of B between 20 s and 30 s is __________ mol/s.
A) +7.3 × 10-3
B) +1.5 × 10-3
C) -7.3 × 10-3
D) +5.0 × 10-4
E) -1.5 × 10-3
7)
8) The average rate disappearance of A between 20 s and 30 s is __________ mol/s.
A) 1.6 × 10-2
B) 0.15
C) 670
D) 1.5 × 10-3
E) 5.0 × 10-4