Let's solve the problems in the image step by step:

25. $20.25 + (-15.711)$

This involves adding a positive and a negative number. It's equivalent to subtracting the absolute value of the second number from the first.

$20.25 + (-15.711) = 20.25 - 15.711 = 4.539$

26. $8.34 + (-9.16)$

Again, adding a positive and a negative number, which is like subtracting.

$8.34 + (-9.16) = 8.34 - 9.16 = -0.82$

29. $10.9 + (-15.6) + 2.1$

First, combine the first two terms:

$10.9 + (-15.6) = 10.9 - 15.6 = -4.7$ Now, add 2.1:

$-4.7 + 2.1 = -2.6$

30. $0.65 + 2.75 + (-1.35)$

Start with the first two terms:

$0.65 + 2.75 = 3.4$ Now, add the third term:

$3.4 + (-1.35) = 3.4 - 1.35 = 2.05$

Let me know if you'd like further details or explanations on any of these steps.

Here are five related questions to expand your understanding:

1. How do you handle addition of negative numbers in general?
2. What is the result of $(-5.2) + 6.8$?
3. How do you simplify expressions with both addition and subtraction of decimals?
4. Why is subtracting a negative number the same as adding its absolute value?
5. How would you compute $12.5 + (-8.7) + (-3.4)$?

Tip: When working with negative and positive numbers, treat negative signs like directions: subtracting a negative moves you "forward" while adding a negative moves you "backward."