Question

If a driver had 6 unpaid parking tickets and must pay $15 per ticket, what is the change in the driver's bank account after paying the fines?

Answer 1

`- $90 larr" negative value"`

Explanation:

`color(blue)("Answering the question")`

`color(green)("6 of tickets")`

but each ticket is worth $15 so we have

`color(green)(6" of "$15)color(brown)(" " ->" " 6xx$15 =$90)`

We now know the quantity but this has to be expressed as a change.

The funds in the account reduce so the change is negative in that the funds are now:

`"Original sum" - $90`

So the change is `-$90.00`
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
`color(blue)("Trick for multiplying by 15")`

15 is the same as 10 + 5

but 5 is half of ten

So multiply in such a way that you have

`6xx15`

`6xx(10+5)`

Multiply everything inside the bracket by 6

`6xx(10" "+" "color(red)(5))`

`(6xx10)" " +" "(6xxcolor(red)(10/2))`

`(6xx10)" " +" "((6xxcolor(red)(10))/(color(red)(2)))`

`" "60" "+" "30" "=" "90`