请教Gmatprep的数学题

gstar

1 A certain business produce x rakes each month from November through February and shipped x/2 rakes at the beginning of each month from march through October.The business paid no storage costs for the rakes from November through February,but it paid storage costs of $0.1 per rake each month from March through October of the rakes that had not been shipped.In terms of x,what was the total storage cost,in dollars，that the business paid for the rakes for the 12 monthd from November through October?

(A)0.4x

(B)1.2x

(C)1.4x

(D)1.6x

(E)3.2x

KEY:C

I choose D4x/2)*0.1*8,why D is wrong?

2 Is the integer n odd?

(1)n id divisible by 3

(2)2n is divisible by twice as many positive integers as n

KEY:B

我第二个条件没看懂,是什么意思呀?

welldone2007

1.等差数列

A3=3.5x

A4=3x

A4=3.5x

…

 

A10=a3+ (n-1)d =a3+7d=3.5x-7*0.5x=0

S=(a3+a10)*d/2 =14x

Storage cost = 0.1S=1.4x

2) looks like “sentence correction”. It omits some: 2n is divisible by twice as many positive integers as n (is divisible by positive integers).

Suppose n=a * b * c * d(分解因子)
            。If n is even, then at least one out of a, b, c, d is even. Then 2n=2(a*b*c*d). It’s possible 2a or 2b, or 2c or 2d equal to that even 因子. ----“not twice”.

If n is odd, then a, b, c ,d  won’t be even. 2a, 2b, 2c, 2d won’t equal to any 因子. ----so twice

gstar

豁然开朗了，万分感谢！

welldone2007

以下是引用welldone2007在2007-5-4 22:18:00的发言：

1.等差数列

A3=3.5x

A4=3x

A4=3.5x

…

A10=a3+ (n-1)d =a3+7d=3.5x-7*0.5x=0

S=(a3+a10)*d/2 =14x

Storage cost = 0.1S=1.4x

2) looks like “sentence correction”. It omits some: 2n is divisible by twice as many positive integers as n (is divisible by positive integers).

Suppose n=a * b * c * d(分解因子)
   。If n is even, then at least one out of a, b, c, d is even. Then 2n=2(a*b*c*d). It’s possible 2a or 2b, or 2c or 2d equal to that even 因子. ----“not twice”.

If n is odd, then a, b, c ,d  won’t be even. 2a, 2b, 2c, 2d won’t equal to any 因子. ----so twice

found erros, sorry mistyped. S = (a3+A10)*n/2 = 14x