MATHS Help please...We're stuck on some 11+ questions

[Ingles2]

Please can someone show ds1 and myself how to work out this equation question
4n+15=45-2n
ds thinks the answer is 15,
the book says 5 and
I'm clueless how to even start...

If 4n + 15 = 45 - 2n

then add 2n to each side to give

6n + 15 = 45

then subtract 15 from both sides to get

6n = 30

and divide by 6 to get

n = 5

Voila!

take 15 from both sides
4n = 45-15-2n

4n = 30-2n

add 2n to both sides
6n = 30

n=5

x=posts! hope you fathom it!

Bella are you there.... we need help going through it...please xx

What do you need help with?

pinky
you're closer to ds method
he had
4n=30-2n
but then he took away 2n so
4n =30 and then took 2n from 4n
so 2n=30
why is iit 4n+2n not 4n -2n????

what do you need help with?

the general rule with equations is that you can do anything you like as long as you do it to both sides.

so you pick something that will help simply fy one of the sides and then so the same thing to the other side as well.

as the others have said, in this case, adding '2n' and subtracting 15 are the best things to do.

I'm here!

The first step is fine (just doing it in a different order to me, but fine)

but if you have -2n on one side then you have to ADD 2n to both sides to get rid of it

(I don't like to say 'move it to the other side' becuase what you are really doing is the same thing to both sides to keep the whole thing balanced)

you need to add 2n not take it away.

if you take 2n away from -2n, you'd get -4n.

Yes do anything you like but to both sides.

remember if you take away from a minus then it gets more minus so -2n take away 2n is actually -4n. If you have -2n and you want to get rid of it, you'll have to ADD 2n.

I'll check back in a min so if you need more help just spell it out!

He's got it!
Thankyou Thankyou...
I have more questions... would you mind???

If you take away 2n from 30-2n you will have 30-4n, ( 30-2n-2n) so to get rid of the -2n on the RHS you need to add on 2n to leave the 30 on its own. If you add 2n to the RHS you must also add 2n to the LHS so you end up with 6n here. So 6n=30

Ooh, I love algebra, off you go!

I have a percentage question...
I'm trying to explain but am definitely making it worse..
The question is

A TV had had it's price reduced by 20% in a sale
The sale price of the TV is #160
What was the original price?

This is how they taught us in the dark ages.

4n+15 = 45-2n

4n+2n = 45-15

6n=30

n= 30/6 = 5

basically it was if you change the side you change the sign.

£160 represents 80% of the original price.

Ask him how he would find 10%, if he knows 80%.

And then ask him how he would work out 100% (ie the orinal cost)

(divide by 8, multiply by 10)

Yes, anything you do you must do to both sides

So

4n + 15 = 45 - 2n

You want to get rid of the "+15" on the left hand side. You get rid of "+15" by subtracting 15, so you subtract 15 from both sides.

4n + (15 - 15) = (45 - 15) - 2n
4n = 30 - 2n

You want to get rid of the "-2n" on the right hand side. You get rid of "-2n" by adding 2n, so you add 2n to each side.

4n + 2n = (30 - 2n) + 2n

6n = 30

You want to get rid of the "6" multiplier on the left hand side. You get rid of multiplying by six by dividing by six, so you divide each side by six

6n / 6 = 30 / 6

n = 5

If it has been reduced by 20% then 80% of the price is £160.

Find one percent, so divide £160 by 80 = £2
make it 100% so times £2 by 100 = £200.

and 1 more algebra question

Boris is t years old
Kyle iis 5 years older than Boris
Ian is 3 times as old as Kyle

What expression give Ian's age

3t
t+5
3t+5
3t+15
5t+3

Lots of eager mathematicians, so I'm off!

Hope he cracks it, Ingles2!

£160 was 80% of the original price.

Work out what 1% of the original price was by dividing by 80:

£160/80 was 80/80%

£2 was 1% of the price

Now you need to know the 100% price so multiply by 100.

£2100 was 1%100

£200 was 100% of the price.

So the full price was £200.

Do a little check at the end:

20% of £200

20/100 * £200 = £40

£200 less the sale discount of £40 = £160.

malalueca - that's a good method once you understand WHY it works.

But leads to children doing things like taking away part of the top of a fraction, instead of multiplying first.

Boris: t
Kyle: t + 5
Ian: 3 * (t + 5) = 3t + 15

yes - I think you are correct in that bella. Also we didn;t do that kind of stuff for 11+, we did it at high school.

Boris is t years old

Kyle iis 5 years older than Boris

So Kyle is t + 5 years old

Ian is 3 times as old as Kyle

So Ian is 3(t + 5) years old

3(t + 5) multiplies out to 3t + 15