Answer:
52
Step-by-step explanation:
The missing sides is equal to the opposite side
So add all the sides up
Obviously, 0.8, all you do is move over the decimal.
Isolate the variable by dividing each side by factors that dont contain variables
Answer: x=3
If the series is made up of numbers in an arithmetic progression, you have

where

is the first term and

is the common difference between terms.
Since the last term in the series is

, you have

Solve the system

and you'll find that the first term is

(with

).
30% of 250 can be written as 0.3 * 250:
0.3(250) = 75
30% of 250 is 75. Is this what you mean by "compute with percents?"