Whats the OA? is it 14%

Confirm and then I will let u know how I solved.

Made a mistake.. Got new days = 87% old days,hece 13% is correct.

Hey there.

We need to introduce 3 variables here.
x- rate.
D- days prior to rate increase
n - days reducted.

so, if the monthly price have to be the same, we have an equation :
xD=1.15*x*(D-n), we need to know n, from this equation. "x" may be terminated

1.15 = 1 15/100, 1 3/20 or 23/20

so,

D-n=D*20/23, -n=(20D/23)-D, n=(3/23)D, or 12/92, would be less then 14% but greater than 13 %.
What is the OA?

you got it!13%

alternative way is just to plug in nums:

orig rate =100, new rate = 115
orig days = 2, new days = 200/115 = 40/23

% change = difference/original = (6/23) / 2 = 6/46 -> round it up to 6/48 -> ans is greater than 1/8 since we rounded the denominator up --> ans>12.5%

hope that helps.

I guess the easiest way is to apply inversly prop rule

r * d = amount spent

amount spent is constant. if you increase r by 15%, to maintain the same constant #, we must multiply d by 100/115.

R = r * 115/100

Hence, R * 115/100 * D * 100/115 (must be multiplied to maintain constant) = amount spent

Balance is simple (old - new) / old formula

HTH

Hi:

What about this?
100 -- previous month
115 -- current month

in the current month, I need to save 15, the % to save is 15/115 = 13%

We can't count days 'cause it will change every month (28,29,30-31).

Is this correct? I know this formula is valid for the first month, but, as I see, all the other solutions are valid just for the first month, 13%, am I wrong?

Silvia