79.in a meeting of 3 representatives from each of 6 different companies, each person shook hands with every person not from his or her own company . if the representatives did not shake hands with people from their own company, how many handshakes take place?

ans:135

80.from a group of 3 boys and 3 girls, 4 children are to be randomly selected. what is the probability that equal numbers of boys and girls will be selected?

ans: 3/5

81.the first roll of a cinema has 16 seats, the roll immediately after the first roll has 2 more seats than the first roll. every roll has two more seats than the roll immediately in front of it. the last roll of has 64 seats. how many seats does this cinema have?

ans: 1000

82.in the parallelogram shown, pq=4, qr=6,angleqps is 30 degree, what is the area of pqrs?

q ______________r

/l /

/ l /

p /__ l__________/s

83.a scientist recorded the number of eggs in each of the 10 birds' nests, what was the standard devition of the numbers of eggs in the 10 nests?

（1） the averge （arithmetic mean） number of eggs for the 10 nests was 4

（2） each of the 10 nests contained the same number

ans: b

84.the base of a retangular solid tank is a square, the length of each side of the square is 4 meters, the altitude of the tank is 6, if there is 16 cube meters water in the tank, what is the rate of the volume of water in the tank to the volume of the retangular solid tank?

ans: 1/6

85. is n more than m?

a. n is more than 2m-1 b. m is more than 1