Answer:
5
Step-by-step explanation:
Usually, the sequence is [tex]a_{1}, a_{2}, a_{3}, a_{4}, ..., a_{n}[/tex] where n is a postive number.
From the question,
[tex]a_{n} = a_{n-1} - 5[/tex]
if n = 2 ;
[tex]a_{2} = a_{1} - 5[/tex]
[tex]a_{2} = 20 - 5 = 15\\[/tex]
Thus, we got [tex]a_{2} = 20[/tex].
if n = 3 ;
[tex]a_{3} = a_{2} - 5[/tex]
[tex]a_{3} = 15 - 5 = 10[/tex]
Thus, we got [tex]a_{3} = 10[/tex].
if n = 4 ;
[tex]a_{4} = a_{3} - 5[/tex]
[tex]a_{4} = 10 - 5 = 5[/tex]
Thus, we got [tex]a_{4} = 5[/tex].
***There are many ways to solve this question
example:
[tex]a_{n} = a_{n-1} - 5[/tex]
[tex]a_{n} - a_{n-1} = - 5[/tex]
Substitute n with 2, 3, 4
Then, we will get 3 equations.
Sum them up;
We will get
[tex]a_{4}-a_{3}+a_{3}-a_{2}+a_{2}-a_{1} = -15\\a_{4}-a_{1} = -15\\a_{4} = -15+20 = 5[/tex]
