#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#