Question

Sean says:
'My plan was to add 5 to my number then multiply the result by 3. Then I multiplied the result by 5 and finally subtracted 6. The answer is 129'
What number was Sean first thinking of?

Answer 1

Let's denote Sean's original number as \( x \).

According to Sean's plan, he performs the following steps:

1. Add 5 to \( x \):
\[
x + 5
\]

2. Multiply the result by 3:
\[
3(x + 5)
\]

3. Multiply that result by 5:
\[
5 \times 3(x + 5) = 15(x + 5)
\]

4. Subtract 6 from the result:
\[
15(x + 5) - 6
\]

According to Sean, this final result is equal to 129:
\[
15(x + 5) - 6 = 129
\]

Now, let's solve this equation step by step.

First, add 6 to both sides:
\[
15(x + 5) = 129 + 6
\]
\[
15(x + 5) = 135
\]

Next, divide both sides by 15:
\[
x + 5 = \frac{135}{15}
\]
\[
x + 5 = 9
\]

Now, subtract 5 from both sides to solve for \( x \):
\[
x = 9 - 5
\]
\[
x = 4
\]

Thus, the number Sean was first thinking of is \( \boxed{4} \).