To solve the problem, let's break it down step by step.

### a) Calculate the area of the rectangle.

Given:
- Length (chiều dài) = 60 m
- Width (chiều rộng) = \(\frac{3}{5}\) of the length

First, calculate the width:
\[
\text{Width} = \frac{3}{5} \times 60 = 36 \, \text{m}
\]

Now, calculate the area (A) of the rectangle using the formula:
\[
A = \text{length} \times \text{width} = 60 \times 36
\]
\[
A = 2160 \, \text{m}^2
\]

### b) Find the area of the remaining land.

Given:
- \( \frac{5}{6} \) of the area of the pond is \( 360 \, \text{m}^2 \).

First, calculate the total area of the pond:
\[
\text{Total area of pond} = \frac{360}{\frac{5}{6}} = 360 \times \frac{6}{5} = 432 \, \text{m}^2
\]

Now, find the area of the remaining land by subtracting the area of the pond from the area of the rectangle:
\[
\text{Remaining land area} = \text{Area of rectangle} - \text{Area of pond} = 2160 - 432
\]
\[
\text{Remaining land area} = 1728 \, \text{m}^2
\]

### Percentage of remaining land area:
To find what percentage of the total land area this remaining area represents, use the formula:
\[
\text{Percentage} = \left(\frac{\text{Remaining land area}}{\text{Area of rectangle}}\right) \times 100
\]
\[
\text{Percentage} = \left(\frac{1728}{2160}\right) \times 100 \approx 80\%
\]

### Final Answers:
- a) Area of the rectangle: \(2160 \, \text{m}^2\)
- b) Area of remaining land: \(1728 \, \text{m}^2\) (approximately 80% of the total area)