Answer: B. 6
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Step-by-step explanation:
In math, the term "substitution" basically means "replace". Keep in mind that x is a placeholder for a number. Think of it as a box we place a number inside.
We have the items {6, 16, 24} to use as replacements for x. We only do one number at a time
Let's try x = 6
4 + x < 20
4 + 6 < 20 ... replace x with 6
10 < 20 ... this is a true statement as 10 is less than 20
So x = 6 is a solution
Let's try x = 16
4+x < 20
4+16 < 20
20 < 20 ... this is false because we can't have a number smaller than itself
Lastly, let's try x = 24
4+x < 20
4+24 < 20
28 < 20 ... also false; 28 is not smaller than 20
We've shown that x = 16 and x = 24 are not solutions. Only x = 6 is a solution from the set {6,16,24}. This is why the answer is choice B.
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Alternative method:
4+x is the same as x+4. We can add two numbers in any order we want.
x+4 < 20 solves to x < 16 after subtracting 4 from both sides. So the set of solutions is anything smaller than 16. Looking at {6,16,24} we see that the only allowed answer is x = 6.
x = 16 is not a solution because x < 16 would turn into 16 < 16, but again we can't have a number smaller than itself.