[!!!急]數學_Probility_8

A box contains 156 rubber bands. There are 100 yellow rubber bands and the rest are green or red. The green ones are 3 times as many as the red ones.

a) Find the number of green bands.

Let x be the munber of the red ones. Then, the number of green ones are 3x.
x+3x+100 = 156
4x = 56
x = 14
∴the number of green bands are 14X3=42.

b) Find the probability of getting a red band if a band is randomly chosen from the box.

P(getting a red band)
=14/156
=1/14

c) y red bands are added to the box. The probability of part b) becomes 10/81 . Find y.

(14+y)/(156+y) = 10/81
1134+81y = 1560+10y
71y = 426
y = 6

2009-06-02 21:42:09 補充：
2b 係 7 / 78
打錯左

2009-06-02 21:43:30 補充：
2a. Let x be the munber of the red ones.
應該是Let x be the number of the red ones.

a)
(１５６-１００) x ３/4 = ４２

b)
(１５６-１００-４２)/１５６ = 7/78

c)
(１５６-１００-４２+ y ) / (１５６+ y ) = １０/８１
８１(１４+ y ) = １０(１５６+ y )
１１３４+８１y = １５６０ + １０y
７１y = １５６０ - １１３４
y = ６

a) 42

b) 7/78

c) 6

請問概率係咪串錯左?@@
好似Probability至岩wor