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At the beginning of a program, the 105 members of a marching band stand in a rectangular formation named Formation A. All of the band members then move into Formation B, which is a different rectangular formation with six more rows, but with two fewer band members per row. How many rows are in Formation A?

Guest Jul 6, 2018

#1**+2 **

OK, this is what I think:

Since 105 =3 x 5 x 7 =15 x 7.

So, you would have 15 rows of 7 kids per row in Formation A.

In Formation B, you would have =15 + 6 = 21 rows with 5 kids per row. So that:

15 x 7 = 21 x 5

Guest Jul 7, 2018

edited by
Guest
Jul 7, 2018

#2**+1 **

Let the number of rows of the original formation = R

And let the number of columns = C

So R * C = 105 ⇒ C = 105/R (1)

Now...in the new formation.....the number of rows is ( R + 6) and the number of columns = (C - 2)

So we have that

(R + 6) (C - 2) = 105 .... expand...

R*C + 6C - 2R - 12 = 105

105 + 6C - 2R - 12 = 105 ...simplify...

6C - 2R = 12 sub (1) into this

6(105/R) - 2R = 12 multiply through by R

630 - 2R^2 = 12R rearrange

2R^2 + 12R - 630 = 0 divide through y 2

R^2 + 6R - 315 = 0 factor

(R + 21) ( R - 15) = 0

Set both factors to 0 and solve for R

R = -21 reject

R = 15 accept

So....there were 15 rows in Formation A

CPhill Jul 7, 2018