Question

There is a value of $a$ such that subtracting $a-4$ from $4a+16$ gives an answer of $-25$. What is that value of $a$?

Answer 1

A is -15
4a + 16 - a + 4 = -25
3a + 20 = -25
3a = -45
A = -15

Answer 2

a = -15

Explanation:

Let's rewrite the exercise in math language.

We have to subtract a-4 from 4a+16 to get -25, that is

4a+16 - (a-4) = -25

The minus sign that is just before the parenthesis changes all the signs inside it. Therefore, if we want to delete the parenthesis, we need to change the signs before.

4a + 16 - a + 4 = -25

Now we can associate the terms with an a between them and the terms without it from the left side of the equation.

3a + 20 = -25

Let's subtract 20 on both sides.

3a + 20 - 20 = -25 - 20

3a = -45

Finally, we divide by 3 on both sides

3a/3 = -45/3

a = -15