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# Algebra/Cross products of proportions

Question
Hi I would like to know how to solve the following:

2.2/3.3=a-2/a-1

Any help on this would be great , thanks.

Hello Suzi,

It is a bit ambiguous, since it is not clear if you meant
1. 2.2/3.3=a-2/a-1, or
2. 2.2/3.3=(a-2)/(a-1)

They are quite different!  I'll assume you meant (2), since the subject you indicated is
"cross product of proportions."

First, to simplify the problem, I would simplify 2.2/3.3 to 2/3.
So, this gives: 2/3=(a-2)/(a-1), now "cross multiply" to get:
2(a-1)=3(a-2) ==> 2a-2=3a-6, now subtract 2a from both sides to get: -2=a-6,
then add 6 to both sides: 4=a...so a=4.

Good?

Abe

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Algebra

Volunteer

#### Abe Mantell

##### Expertise

Hello, I am a college professor of mathematics and regularly teach all levels from elementary mathematics through differential equations, and would be happy to assist anyone with such questions!

##### Experience

Over 15 years teaching at the college level.

Organizations
NCTM, NYSMATYC, AMATYC, MAA, NYSUT, AFT.

Education/Credentials
B.S. in Mathematics from Rensselaer Polytechnic Institute
M.S. (and A.B.D.) in Applied Mathematics from SUNY @ Stony Brook