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From this system of equations we find at first C  80, and
then
1
1  1  20
k
e 20k   e    .
2  2 
.
Consequently, the law of change of temperature depending
on time has a kind
t
 1  20
U  20 80   .
 2 

In the considered example at drafting of differential
equation we dealt directly with derivative the sought function
after. We will make an example now, where more comfortable
to conduct reasoning, operating with the differentials of the
sought sizes after. 

Example 2.14 In a reservoir, that contains 10 kg of salt
on 100 l of mixture, every minute 30 l of water disembogues
and 20 l of mixture flows out (fig. 2.1).

Fig. 2.1

To define the amount of salt that will remain in a reservoir
through t min., considering, that a mixture is instantly mixed.

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