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tonebeeze
I got this problem correct using DS guessing theory. I reviewed the answer explanation in the m24, but I still did not fully understand. Can someone flesh out statement 2 for me. Thanks!

Price increased by M% from 1991 to 1992 and by N% from 1992 to 1993. What was the percentage increase in price from 1991 to 1993?

1. MN = 300

2. 100M + 100N + MN = 4300

Hi, would you mind explaining what exactly is DS guessing theory? Could you kindly elaborate more on this?
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tonebeeze
I got this problem correct using DS guessing theory. I reviewed the answer explanation in the m24, but I still did not fully understand. Can someone flesh out statement 2 for me. Thanks!
Price increased by M% from 1991 to 1992 and by N% from 1992 to 1993. What was the percentage increase in price from 1991 to 1993?

1. MN = 300

2. 100M + 100N + MN = 4300

Hi, would you mind explaining what exactly is DS guessing theory? Could you kindly elaborate more on this?
Let price on 1991 be P
in 1992=P(1+m/100)
1993=P(1+m/100)(1+n/100)
%increase= change(1993)-original(1991)/original(1991)
=P(1+m/100)(1+n/100)-P/P
P cancels out.
(1+m/100)(1+n/100)-1
(10^4+100m+100n+mn-10^4)/10^4
(100m+100n+mn)/10^4------(a)
Given in (2) 100m+100n+mn=4300-------->m+n+mn/100=43----(b)
taking 100 out common from (a)
(m+n+mn/100)/10^2
substituting from(b) we get
43/100
43%
Ans B
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There is a formula for cumulative percentage changes:

a + b + ab/100 = total percentage change
a = first period change
b = second period change.

In statement (2) we are simply given this formula, multiplied by 100.

100M + 100N + MN = 4300

equals

M + N + MN/100 = 43

The cumulative increase is 43 %.
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