It turns out that 5% of 400 is 20.
(1/20)*400=20.
So Ian was being paid £400 a week before he was given a pay rise.
Answer:
The sum rule is f' + g'
The difference rule is f' − g'
The product rule is f g' + f' g
The quotient rule is (f' g − g' f )/g2
Divide both sides by 0.32 and you'll get the value for c