The Triangle Inequality Theorem states that the sum of any 2 sides of a triangle must be greater than the measure of the third side. (Think about it, if one side was the same length as the other two combined that would make a straight line. If it was any longer the other two sides couldn't reach the endpoints.)

This means:

(x) + (x + 4) > (20)

2x + 4 > 20

2x > 5

__x > 2.5__

It also means:

(x) + (20) > (x + 4)

and

(x + 4) + (20) > x

but the last two don't help any because they are true for all values of x, so it doesn't narrow down the answer any more.

Now you also said the triangle is acute. That means that all angles are less than 90°. Since 20 is the longest side length, the angle across from the side with length 20 is the biggest angle. So x has to be less than whatever values make that angle exactly 90. So just find the values of x when the angle opposite the side with length 20 is 90.

Pythagorean theorem says:

x^{2} + (x+4)^{2} = 20^{2}

x^{2} + (x+4)(x+4) = 400

x^{2} + x^{2} + 8x + 16 = 400

2x^{2} + 8x + 16 = 400

Divide everything by 2.

x^{2} + 4x + 8 = 200

x^{2} + 4x - 192 = 0

Use quadratic formula to figure out that:

x = 12 and x = -16

It can't be negative because we are talking about side lengths.

So x has to be less than 12. __x < 12__

SO the final answer is **2.5 < x < 12**