Maths 3rd Year Secondary School!Percentages and interest!

Miki150

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This is the question: By the way the answer is 8000euro (the answer was at the back of the book) I just don't know how to get the answer. Thank you!
A person invests B euro in a building society at 4% per annum. At the end of the first year the person invested a further B euro, and left all the money in a society for a further year at 5% per annum. If the total investment at the end of the second year amounted to 17136 euro, find the value of B.

So here is how I would do the question if B was given:
8000 x 4%= 320 + 8000= 8320
8320+8000=16320
16320 x 5% = 816
16320 + 816 = 17136

And here is how I tried to work it out to get B but got stuck during the way :)
17136\105 x 100 = 16320
17136 - 16320 = 816 also 16320 x 5 over 100 = 816
so that means: B + B + (B x 4%) = 16320
So how mathematicaly doing the calculations can you get 8000. I am so confused. Thank you so much! :)
 
This is the question: By the way the answer is 8000euro (the answer was at the back of the book) I just don't know how to get the answer. Thank you!
A person invests B euro in a building society at 4% per annum. At the end of the first year the person invested a further B euro, and left all the money in a society for a further year at 5% per annum. If the total investment at the end of the second year amounted to 17136 euro, find the value of B.

So here is how I would do the question if B was given:
8000 x 4%= 320 + 8000= 8320
8320+8000=16320
16320 x 5% = 816
16320 + 816 = 17136

And here is how I tried to work it out to get B but got stuck during the way :)
17136\105 x 100 = 16320
17136 - 16320 = 816 also 16320 x 5 over 100 = 816
so that means: B + B + (B x 4%) = 16320
So how mathematicaly doing the calculations can you get 8000. I am so confused. Thank you so much! :)
1.04B = the amount at the end of the first year.

(B + 1.04B)(1.05) = 17136

2.04B * 1.05 = 2.142B = 17136

B = 17136/2.142 = 8000

ALL the money includes what was invested at the start of the first year plus the interest earned during that year as well as what was invested at the start of the second year..
 
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