Question 729828
always try to substitute the word "percent" or sign "%"  by(*(1/100)) that means you divide that number by 100, will help a lot,
so 15% or 15 percent is now:
{{{15*(1/100)}}} or {{{15/100)}}} or {{{0.15}}} so,
the discount part is just 0.15 (let's name it "desired part")that means you should still had to pay for the rest which is {{{1-0.15=  0.85}}}.  let's name this as "undesired part"
Again the key to understand this type of questions is that all part of things or values are equal to one {{{1}}}  which consist of "desired and undesired part".
So, if you get a discount as 0.15 (desired) the rest would be 0.85 (undesired) to pay.
To figure out how much discount you are going to get, just multiply whole value (40 pounds) by the desired part  which means : {{{0.15*40=6}}} and the amount you have to pay {{{0.85*40=34}}} pounds, respectively.

{Another example: if you have 20 dishes and break 25 percent of them by accident (25/100) (0.25 undesired) that means the rest of (0.75 desired) are still O.K. the (whole was 1) (faulty 0.25) (o.k. 0.75)} so,
{{{0.25*20=5}}} are useless and
{{{0.75*20=15}}} of them are still there for you to break them and have a relief, just joking.

hope that helps