12个笔, 三个坏的.   取两支,  求没有坏的的概率.

from A to B,   20 miles per gallon of gas,   from B to A, different route, 25 miles per gallon of gas.  ask   average  how  many gallon of gas per mile  for the round trip.
1.  round trip 175 miles
2. from B to A  the distance  is 3/4 of  distance from A to B.


sweep floor.   A takes 3 times as long as B.  A and B together  needs  24 mins. ask  how long  does B ( or A)  need to do it  alone

Need to put many boxes into warehouse.  All box are the same.  The warehouse  area is 1200 sq feet.  Height 12 feet.   The box can be stacked, but the arrow drawn on it must  be upwards.  Ask if  all the box can be placed into warehouse.

1. the volume of each other box is 60.
2. there are total 240 boxes

Is |s-t| > ||s|-|t||?

1. st<0
2. s-t < 0

A person rounds each item on the telephone bill charge to the nearest dollar.  Ask if the final total after rounding is within $8 of the actual total

1.  each item is < $7.75 on the bill

2. there are total 14 items  on the bill

Another question on telephone bill.  Every item is rounded to the nearest dollar.  Then add them up to get an estimated amount.  Ask under what condition,  is the estimated amount  > the actual total.

1. half of the items  is rounded  UP to the whole dollar.
2. there are 17 items in the bill.

Set A has n integers. Arithmetic mean is m. standard deviation is R.   Set B has all the elements of A  and  x.  if  standard deviation of B is r ,   is r < R?

1. x = m
2. m <= x <= m+R

there are tree seeds  numbered from 201 to 740. Among these seeds,  the hundred digit is 4, 5, 6 or 7.   and  unit digit is 2, 3, 4 or 5. are  pine tree seeds.  Ask if select one seed randomly, what is the probability of picking  a non-pine tree seed.

X^2 + bX + c = 0. has two different roots  what is the product of these two roots?

1. one root is 3
2. c = 4.