Answer
[ 1 by 2, -1 by 2 ]
Step-by-step explanation:
https://haygot.s3.amazonaws.com/questions/1118559_1200733_ans_2ec06a240d2240e49dc1c9fda7b41ef5.jpeg
The unknown number is 1/2 or 0.5.
ex: 4x+6=8
For this case we have the following polynomials:

To divide the polynomials we must follow the following steps:
1) Same base, the exponents are added.
2) Only terms that have the same exponent can be added or subtracted.
3) If the remainder is zero, the division is exact.
Answer:
Note: see attached image for the solution.
1ml of 80ml bottle: 55/80 = 0.6875 pence
1ml of 50ml bottle: 45/50 = 0.9 pence
If no special offer the first bottle is a better value for money.
Since special offer though, you'd be buying;
50ml bottle £45 + another 50ml £22.5 = £67.5 in
which case 1ml would cost;
67.5/100 = 0.675 pence. Hence buying 2x50ml
bottle would be a better value.