Toán tiếng Anh lớp 7

To solve the problem, we need to find how many sheets of tissue paper are in a pack based on the given information.

### Given Data:
1. Dimensions of one sheet of tissue paper:
- Length = 138 mm = 13.8 cm
- Width = 100 mm = 10 cm
- Layers = 3 (3 ply)

2. Thickness of each tissue paper = 0.13 mm = 0.013 cm

3. Dimensions of the cylindrical tissue roll:
- Diameter of the rolled tissue paper = 12 cm
- Diameter of the cardboard roll = 5 cm

### Steps to Find Number of Sheets:

1. **Calculate the radius of the layers of tissue paper:**
The diameter of the rolled tissue paper is 12 cm, so the radius (\(R\)) is:
\[
R = \frac{12}{2} = 6 \text{ cm}
\]

The diameter of the cardboard is 5 cm, so the radius (\(r\)) of the cardboard is:
\[
r = \frac{5}{2} = 2.5 \text{ cm}
\]

The thickness of each layer of tissue paper is 0.013 cm.

2. **Find the thickness of the tissue paper layers:**
The total thickness of the tissue paper when rolled around the cardboard can be expressed in terms of its layers. The number of layers of tissue paper is given by the difference in radius divided by the thickness of one layer:
\[
\text{Number of layers} = \frac{R - r}{\text{Thickness of one sheet}}
\]
Substituting the values:
\[
\text{Number of layers} = \frac{6 - 2.5}{0.013} \approx \frac{3.5}{0.013} \approx 269.23 \text{ layers}
\]

Rounding down, we find that there are approximately 269 layers of tissue paper rolled.

3. **Calculate the number of sheets:**
Since each sheet consists of 3 layers (3 ply), the number of actual sheets (\(N\)) is:
\[
N = \frac{\text{Total layers}}{\text{Number of layers per sheet}} = \frac{269}{3} \approx 89.67
\]

Rounding down, we have approximately **89 sheets** of tissue paper in a pack.

### Final Answer:
There are approximately 89 sheets of tissue paper in a pack.