top urgent! help!

happy2000up

1. K is 2-digit positive integer, the value of K is 6 times of the unit digit. K=?
1) the tenth digit is 4 greater than unit digit.
2) the sum of the two digits is a 2-digit integer.

JJ ans. is D. I don't know why? Pls help.


60 cars, either full-size or compact or sports. 31 white, 10 red, 19 blue. How many cars are full-size?
1) Not all red and blue are full-size;
2) The number of full-size is greater than the sum of compact and sports cars.

JJ ans. is C. How and why?



SB use 24 pounds of A and 25 pounds of B to make X and Y. X contains A:B=4:1; Y contains A:B=1:5. X is how many pounds?


(x.y)一对数，属于-9 到 10， 问一共有多少组数，并满足x>y?
JJ ans. is C20,2.


更简单的一题，我都不好意思问了-----
X+Y=？
1) 2^X+8^Y=256
2) 3^2X+9^Y=81
JJ ANS IS B.....................WHY? HELLLLPPPPPP!


字母DIGIT组合数，要求I不能在一起。

happy2000up

thank you, pretty girl---louisa.

Louisa_xy

1. K is 2-digit positive integer, the value of K is 6 times of the unit digit. K=?
1) the tenth digit is 4 greater than unit digit.
2) the sum of the two digits is a 2-digit integer.
条件1)好象有问题

Louisa_xy

X+Y=？
1) 2^X+8^Y=256
2) 3^2X+9^Y=81

1) X+3Y=256    X+3Y=8, there are three pairs of X and Y: (2,2) and (5,1),but(8,0) cannot suffice2^X+8^Y=256.  so 1) alone is insufficient
2)2X+2Y=4 there are three pairs of X and Y: (0,2) (2,0) and (1,1), but (0,2) (2,0) do not suffice 3^2X+9^Y=81, thus (1,1) is the only solution.
2) alone is sufficient

Louisa_xy

(x.y)一对数，属于-9 到 10， 问一共有多少组数，并满足x>y?
从-9 到 10这20个个数里选两个C20,2， 这两个数是一个大一个小，让大数为x，小数为y
就只有一种顺序所以最后结果就是C20,2

或者： 当x=10，y可以从19个数选；当x=9，y可以从18个数选。。。当x=-8，y只能是-9
（x，y）的对数是一个递减的等比数列：19+18+17+.....+1=C20,2

Louisa_xy

SB use 24 pounds of A and 25 pounds of B to make X and Y. X contains A:B=4:1; Y contains A:B=1:5. X is how many pounds?

设X 有m pounds，X的4/5是A
  Y 有49-m pounds，A占1/6
4/5*m+（49-m）*1/6=24   m=25

happy2000up

各位帮下手！小生有礼了。