Solving this problem involves a lot of substitutions....at least the way I did it.
Here's what we have
Q1 + P1 = 500
Q2 + P2 = 500
Q2 = 1.3333Q1
Q2-P2 = 2(P1-Q1) and P2 = Q2 - 2(P1-Q1)
Now
Q1+P1=Q2+P2 since they both equal 500
Now the substitution begins
Q1+P1 = 1.3333Q1 + Q2-2(P1-Q1)
Q1+P1 = 1.3333Q1 + 1.3333Q1-2(P1-Q1)
Since all my subscripts are now 1, I'm going to drop the subscript since formatting so many is a bit of a pain.
Q+P = 1.3333Q + 1.3333Q -2(P-Q)
Q+P = 2.6666Q - 2P + 2Q
Q+P = 4.6666Q - 2P
Bring the P terms to the left and the Q terms to the right
3P = 3.6666Q
P = 1.2222Q
Back to the original equation
P + Q = 500
1.2222Q + Q = 500
2.2222Q = 500
Q = 225 This is Q1, the old Q
P = 500-225 = 275 This is P1, the old P
Q2 = 1.3333Q1
Q2 = 1.3333(225) = 300 <===== There are 300 Q seats in the new parliament
P2 = 500-300 = 200
Note that P1-Q1 = 275-225 = 50
and
Q2-P2 = 300-200 = 100
and 100 = 2(50)