1)
Solving from system of equations:
x- amount of 1 cent stamps
y- amount of 33 cent stamps

2)
x- amount of 20 cent stamops
y-amount of 34 cent stamps
From system of equations:
SA=2lw+2lh+2hw ..........
Multiply all of those and minus and do what is said and of not serch for links ok
By moving the signs he actually changed the number itself. He changed -6 from a negative to a positive and 5 from a positive to a negative. Both equations give different answers.
Hopefully this helps :)
theoretical probability is a coin will land on heads 50% of the time
50% of 100 = 50
since it only landed on heads 38 times, this was less than the theoretical probability of 50