#color(blue)("Preamble")#
The symbol % is a bit like a unit of measurement but one that is worth #1/100#
So, in the same way we would say #6" of "x->6x#
We can say #40" of "%->40%=40xx1/100=40/100#
This compares very well to (picking some value at random) #3cm#
centi #->1/100#
3 centimetres #-> 3xx# centi of 1 metre #->3xx1/100xx1" metre"#
#= 3/100" metres"#
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#color(blue)("Answering the question")#
#color(brown)("Breaking the question down into its component parts")#
#40% #of ...........................#40/100xx?#
Let the unknown value be represented by #x# giving:
#40% #of ...........................#40/100xx x->40/100 x#
10 is ....................................#10=?#
10 is 40% of........................#10=40/100x#
Multiply both sides by #color(red)(100/40)#
This changes the #40/100# on the right into 1 so 'getting rid' if it, on that side.
#color(green)(10color(red)(xx100/40)color(white)("d")=color(white)("d")40/100color(red)(xx100/40)xx x)#
#100/4=x#
#x=25#
