Algebra

Just remember letters like x, y, and z are representing numbers

so for example D=S x T is algebra

in this case D is distance

S is speed

T is time

So if a man walks at 4 Kilometres per hour how far (distance) does he walk

in 3 hours.

Well we have the equation D=S X T

and putting the numbers in gives

D=4km/h X 3 Hours

Answer: D=12 kilometres

For lots more detail Wikepedia - Algebra gives a good overview