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It appears that the image contains several math problems. Let's go through each question and solve them step by step.

### Question 28:
**Problem**: 
A metal alloy weighing 40 kg contains 10 kg of iron. If the amount of tin decreases by 12%, how many kilograms of tin were there originally?

**Solution**:
Let \( x \) be the initial amount of tin in the alloy. The alloy originally contains 40 kg, with 10 kg of iron, so the remaining \( (40 - 10) = 30 \) kg is tin. The amount of tin decreases by 12%, meaning the remaining amount of tin is 88% of the original.

The relationship can be modeled by:
\[
30 \times 0.88 = x
\]
Solving this equation will give us the original amount of tin.

### Question 29:
**Problem**: 
Find the transformation matrix of a triangle that is reflected.

**Solution**:
This is a problem on matrix transformations. By looking at the diagrams and comparing the original and reflected triangles, we can deduce that the reflection matrix is likely one of the options given. Based on the image, we can compare the relative positions and conclude which matrix represents the reflection.

### Question 30:
**Problem**: 
The first term of an arithmetic progression is 51, and the common difference is -4. Find the sum of the first \( n \) terms when the sum is maximized.

**Solution**:
The sum of the first \( n \) terms of an arithmetic progression is given by:
\[
S_n = \frac{n}{2} \times (2a + (n-1) d)
\]
where \( a \) is the first term, \( d \) is the common difference, and \( n \) is the number of terms. We need to maximize this sum.

### Question 31:
**Problem**: 
Vectors \( \vec{a} = (-2, p, -3p), \vec{b} = (4, p+3, 3+p), \vec{c} = (2, p-2, q) \). If \( \vec{b} - \vec{a} \) is parallel to \( \vec{c} \), find \( p \) and \( q \).

**Solution**:
For two vectors to be parallel, their cross product must be zero. We can calculate the difference \( \vec{b} - \vec{a} \) and set up the equation such that \( \vec{b} - \vec{a} \) is proportional to \( \vec{c} \).

### Question 32:
**Problem**: 
Evaluate the integral \( \int |x-3| dx \).

**Solution**:
The absolute value function splits the integral into two parts. We can calculate the definite integral by considering the points where \( x = 3 \), and split the integral accordingly:
\[
\int_{a}^{b} |x-3| dx
\]

### Question 33:
**Problem**:
The perimeter of an equilateral triangle inscribed in a circle of radius 2 is \( 4 + 2\sqrt{5} \). Find the area of the triangle.

**Solution**:
The perimeter of the equilateral triangle is given, and we can use the formula for the area of an equilateral triangle, which is:
\[
A = \frac{\sqrt{3}}{4} \times s^2
\]
where \( s \) is the side length, derived from the perimeter.

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Зэс ба төмрийн 40 кг хайлшинд 10 кг төмөр нэмэхэд зэсийн эзлэх хувь нь 12 хувиар багассан бол хайлшинд хэдэн килограмм зэс байсан бэ?

Learn how to solve a percentage decrease problem where an alloy of 40 kg contains 10 kg of iron, and the amount of copper decreases by 12%. We calculate the original amount of copper in the alloy.

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