need homework help! its maths!whats the interest of £320 invested at 4% p.a for a period...?

Question

Calculate the simple interest on £320 invested at 4%p.a for a period of 6 months.
help! me and my friend both have different answers...total different answers.
i got £6.40, and she got £76.80
??????????????

Answer 1

i can't remember how to do this lol.... i know the more complicated one with compounding and stuff.... let me think...

I=Prt

I=320(.04)(.5)= 6.4

the mistake your friend made was that she said 6 instead of .5, you have to do the time in years...

Answer 2

You are right. 320 X .04 (4%) = 12.80 per annum. Six months (one-half year) is one-half of the 12.80 figure for the whole year. 6.40 is correct.
The answers to a lot of math questions like this have to pass the common-sense test at the end, so do it in the beginning. For example, figuring 10% of anything is easy to do in your head (just move the decimal point of the whole number over one space to the left). So if the annual interest rate is 10% on 320 (dollars or pounds), then the amount of interest is going to be 32 dollars/pounds (moving the decimal point in 320.00 one space to the left is 32.0) if you lend out your money for a whole year. Following that along (still in your head), half of that amount for the six months would be 16, then, since 4% is not even half of 10%, the interest you will receive at 4% is going to be less than half of the 16, which means it will be less than 8. After the common sense exercise, do the math on paper for the exact amount: 6.40. (You couldn't possibly receive 76.80 for half that amount of time (six months) and at an interest rate (4%) that is less than half the 10% interest rate you just figured out in your head, could you?)
You got it right - good job.
But I'd certainly love to open a savings account at Rosemary's bank - :)

Answer 3

First, find 4%.

x/320 = 4/100

Use cross multiplication.

100x = 4 X 320

100x = 1,280

100x/100 = 1,280/100

x = 12.8

Next, multiply that by 6, representing the # of months.

12.8 X 6 = 76.8

Therefore, the simple intrest is 76.8.

Note: you do not have to use time in years.