Study the information carefully to answer the question that follow.
In a college there are 900 students who are doing Post Graduation (PG) in any one of the subjects, out of five different subjects, out of the five different subjects viz. Zoology, Botany, Mathematics, Physics and Statistics. The ratio between the boys and the girls among those is 5:4 respectively, 20% of the total girls are doing PG in Zoology and 25% of the total girls are doing PG in Statistics. Total number of students doing PG in Botany is 220. Total students who are doing PG in Mathematics is 150. Respectively ratio between the number of girls and the number of boys doing PG in statistics is 2:3. Twenty percent of the total number of boys are doing PG in Mathematics is 1:2 respectively. There are equal number of boys and girls who are doing PG in PG in Physics. 180 students are doing PG in Zoology.
What is the total number of students during PG in Physics and Statistics together?

A pipe can empty a tank in 40 minutes. A second pipe with diameter twice as much as that of the first is also attached with the tank to empty it . The two pipe together can empty the tank in: 

A policeman starts to chase a thief. When the thief goes 10 steps the policeman moves 8 steps and 5 steps of the policeman are equal to 7 steps of the thief. The ratio of the speeds of the policeman and the thief is: 

If  $$ \ {xy\over x+y}= a, $$ $${xz\over x+z}=b $$ and $$ {yz\over y+z} =c $$  where a, b, c are all non – zero numbers, then x equals to

If a+b+c=15 and a²+b²+c²=83 then the value of a³+b³+c³-3abc