A tank contains 40 gallons of a solution composed of water and bleach. At the start there are 4 gallons of bleach in the tank. A second solution containing half water and half bleach is added tothe tank at the rate of 4 gallons per minute. The solution is kept throughly mixed and drains from the tank at the rate of 4 gallons per minute. (a) How much blcach is in the tank after t minutes?
(b) Will there be at least 14 gallons of bleach in the tank after 10 minutes?
(c) How much bleach will there be in the tank after a very long time has passed?

a) x* e∧ t/10  =  20 *e∧ t/10 + 2

b) x  =  14,2 gall. In 10 min. we get 14,2 gallons of bleach in the tank

c) After a very long time we will find 20 gallons of bleach in the tank

Step-by-step explanation:

The variation of bleach in water in the tank is Δ(x)t:

Δ(x)t  = [Amount of x added to the tank - Amount of x drain out of the tank]*Δt

Now the amount of x added to the tank is:

Rate of adding* the concentration

Rate of adding =  4 gallons per minute

Concentration  half and half ( water + bleach)  = 0,5

Rate of draining =  4 gallons per minute

Concentration of draining: Unknown but can be expressed as:

x(t)/40. According to this

Δ(x)t  =  [4 * 0,5  -  4 * x(t)/40]*Δt

Dividing by Δt on both sides of the equation and tacking limits we get

dx/ dt    =  2  - x / 10

dx/dt + x/10 =2

Multyiplying by e∧ t/10 on both sides

e∧ t/10* [ dx/dt  +  x/10 ]  =  2 *e∧ t/10

To solve it, integrating the first member is

x* e∧ t/10  =  20 *e∧ t/10 + C

for initial condition t = 0

4    =  20 + C

C = -16

a) x* e∧ t/10  =  20 *e∧ t/10 - 16

b) in 10 minutes

x* e∧ t/10  =  20 *e∧ t/10 - 16

x *e  =  20*e  - 16

x   =   20  -  16/e

x   =  20  -  16/2,718

x   =   20 -  5,88

x  =  14,2  gallons.

c) After a very long time we will find 20 gallons of bleach in the tank

this story was prepared by jim

mccullaugh in los angeles and

al stewart in new york.

new york the colossal success of "top gun" appears to have dramatically altered madison avenue's

perception of corporate sponsorship

of home video projects.

the paramount home video re- lease, featuring a 60- second diet

pepsi spot, was by no means the

first prerecorded video to carry an advertiser's plug, but it was the

first major theatrical release to do so. in the aftermath of the title's

success, paramount is widely ex- pected to ink a similar deal for

"crocodile dundee" or "star trek

iv" or both.

corporate america's new attraccds push british

recording sales

to 19% gain

london fueled by the compact

disk boom, the value of u.k. manu- facturer shipments of recordings

rose 19% in the 12 -month period ended march 1987, according to the

british phonographic industry.

total value of u.k. shipments for

the period was $755 million for all

configurations. (all value figures

are based on an exchange rate of

$1.68 to the pound sterling.)

cd sales totaled 10.7 million units

worth $121.3 million- compared

with 3.7 million units worth $39.5

million in the prior 12 months -and

accounted for two -thirds of the

overall gain in trade value. at present growth rates, says the bpi, cd

sales for calendar 1987 will reach 20

(continued on page 79)

step-by-step explanation:

the 3rd option is right

step-by-step explanation: