# GCSE Maths Exponentials

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#1
Hi so I’m stuck on this exponential question, apparently the answer is 81/16. Please could someone tell me what I’ve done wrong and how to correct myself, here’s a pic of my method and the question
Last edited by Mavs04; 3 days ago
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3 days ago
#2
What value of 'Ans' did you enter into your calculator? Otherwise your working is correct.
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3 days ago
#3
I guess they want you to do
3/2
As the ratio, rather than 1.5. So the ans is
(3/2)^4
Last edited by mqb2766; 3 days ago
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3 days ago
#4
(Original post by Mavs04)
Hi so I’m stuck on this exponential question, apparently the answer is 81/16. Please could someone tell me what I’ve done wrong and how to correct myself, here’s a pic of my method and the question
It's like compound interest!!!
At start of day 6 (5 days complete), number of bacteria will be:
1000 x (1.5)^5 (Let's call this value x1)
At start of day 10 (9 days complete), number of bacteria will be:
1000 x (1.5)^9
and we are told this is k times the first value, x1.
So, 1000 x (1.5)^9 = k x 1000 x (1.5)^5

Rearranging to isolate k:

k = 1000 x (1.5)^9
------------------
1000 x (1.5)^5

Most terms cancel top/bottom, leaving

k = (1.5)^4

and if we put 3/2 instead of 1.5 (one and a half)

k = (3 )^4
--------
(2)^4

= 81/16 q.e.d.
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