Given:
15 is decreased by three times a number, the result is 21.
To find:
The equation for the given statement and solve it.
Solution:
Let n be the unknown number. Then,
Three times of a number = [tex]3n[/tex]
15 is decreased by three times a number = [tex]15-3n[/tex]
It is given given that the result after subtraction is 21. So,
[tex]15-3n=21[/tex]
[tex]-3n=21-15[/tex]
[tex]-3n=6[/tex]
Divide both sides by -3.
[tex]n=\dfrac{6}{-3}[/tex]
[tex]n=-2[/tex]
Therefore, the required equation is [tex]15-3n=21[/tex] and its solution is [tex]n=-2[/tex]. Hence, the correct option is (a).
