Question 29611
7. Suppose the amount of passengers boarding in Phoenix is 'x'. In Dallas we 've got the next amount: x - 1/2x + 4/5x (the original with amount with half of them leaving -1/2x and 4/5 of the original amount boarding). Let's call this new amount 'y'. In Memphis we are left with 3/4 of passengers: 
1 - 3/4 = 1/4, but twice the amount of the remaining number boards: 1/4 + 2/4 = again 3/4. We now have 3/4y - passengers. Let's call this number 'z'
In Chicago 2/3 got off: z - 2/3z = 39, leaving 39 passengers.
Now we can calculate: 3/3z - 2/3z = 1/3z = 39 => z = 117. 
If 'z' is 117, y = 4/3z (since 3/4y = z) = 156
We still have to find out about the original number 'x' => x - 1/2x + 4/5x = y = 156 =>

10x - 5x + 8x = 1560

13x = 1560

x = 120


8. 2 unknown variables here, the number of students 'x' and the number of cookies, 'y'. This should give us 2 equations, and indeed:

If we take from the total amount of cookies y a quantity equal to 3 pieces per student -3x, we end up with 5 cookies. If we do the same exercise with 4 cookies per student, we end up in debth, heating the cooker again for 2 more cookies.

y - 3x = 5
y - 4x = -2

Solving:

y + 2 = 4x
(y + 2)/4 = x

y - 3(y + 2)/4 = 5
y - 3/4y - 6/4 = 5
4y - 3y - 6 = 20
y = 26

Leaving us with x = 7


9. 4.327 - 2.500 = 1.827 profit

Buying a camper with 2/3th of 1.823 = 1.218 cost of the camper, leaving only 609 as profit.
Selling the camper for twice the price he paid (1.218) gives a return of 2.436, leaving again a profit of 1.218. Together with the first profit (609), the new amount of profit is $1.827. 

10. 

if 2cm equals 5ft, then 4.8 equals 5x2.4 = 12 and 6 equals 15, leaving us with a room of 180 square feet