Question

-13 is 4 less than a number n

Answer 1

-13 is 4 less than a number n

n - 4 = -13 

Answer 2

Think of it like this.  Do you know what 4 less than a number n is?  Say n=5.  What's 4 less than 5?  That's 1.  What's 4 less than, say 7?  3.  We can write an expression: n-4.

So, if -13 is 4 less than n, or n-4, we have an equation: n-4 = -13 .  Are you familiar with negative numbers?  Well, what's 1-2?  That's -1.  Why?  Well, that's equal to 1-(1+1), which is equal to 1-1-1.  1-1=0, so this simplifies to 0-1.  Can you get any lower than 0?  Yes: using negatives!  0-1 equals 0 + -1.  Just like 0+1=1, 0 + -1 = -1.

Anyway, now we know about negative numbers.  Just think of -13 as 13 less than 0.  So we have n-4 = -13.  This could be your expression, if that's all you need.

Now, to solve, we need to find n, so we need to isolate it by adding 4 (the inverse operation) to both sides.  n-4+4 = -13+4.  n-4+4 equals n, but what's -13 + 4?  Well, if you're 13 below 0, but then you add another 4, then...you're 9 below 0, or -9!  So, n = -9.

Answer: n-4 = -13 OR n = -9