The denominators of the given fractions are $3$ and $6.$ Thus to find the least common denominator, we find the least common multiple of $3$ and $6.$

Let's list the multiples of each number.

Multiples of $3$: $\quad 3,{\color{blue}6},9,12, ...$

Multiples of $6$: $\quad {\color{blue}6},12,18, ...$

We conclude that the least common multiple of $3$ and $6$ is ${\color{blue}{6}}.$

Therefore, the least common denominator of $\dfrac 1 3$ and $\dfrac 5 6$ is $6.$

The denominators of the given fractions are $18$ and $6.$ Thus to find the least common denominator, we find the least common multiple of $18$ and $6.$

Let's list the multiples of each number.

Multiples of $18$: $\quad {\color{blue}18},36,54, ...$

Multiples of $6$: $\quad 6,12,{\color{blue}18}, ...$

We conclude that the least common multiple of $18$ and $6$ is ${\color{blue}{18}}.$

So the least common denominator of $\dfrac 5 {18}$ and $\dfrac 1 6$ is $18.$

The denominators of the given fractions are $4$ and $5.$ Thus to find the least common denominator, we find the least common multiple of $4$ and $5.$

Let's list the multiples of each number.

Multiples of $4$: $\quad 4,8,12,16,{\color{blue}20}, ...$

Multiples of $5$: $\quad 5,10,15,{\color{blue}20},25 ...$

We conclude that the least common multiple of $4$ and $5$ is ${\color{blue}{20}}.$

Therefore, the least common denominator of $\dfrac 3 4$ and $\dfrac 4 5$ is $20.$

The denominators of the given fractions are $7$ and $9.$ To find the least common denominator, we find the least common multiple of $7$ and $9.$

Let's list the multiples of each number.

Multiples of $7$: $\quad 7,14,21,28,35,42,49,56,{\color{blue}63}, ...$

Multiples of $9$: $\quad 9,18,27,36,45,54,{\color{blue}63}, ...$

We conclude that the least common multiple of $7$ and $9$ is ${\color{blue}{63}}.$

So the least common denominator of $\dfrac 2 7$ and $\dfrac 4 9$ is $63.$

The denominators of the given fractions are $10$ and $4.$ To find the least common denominator, we find the least common multiple of $10$ and $4.$

Let's list the multiples of each number.

Multiples of $10$: $\quad 10, {\color{blue}20}, 30, \ldots $

Multiples of $4$: $\quad 4, 8, 12, 16, {\color{blue}20}, \ldots$

We conclude that the least common multiple of $10$ and $4$ is ${\color{blue}{20}}.$

Therefore, the least common denominator of $\dfrac{7}{10}$ and $\dfrac{1}{4}$ is $20.$

The denominators of the given fractions are $6$ and $21.$ To find the least common denominator, we find the least common multiple of $6$ and $21.$

Let's list the multiples of each number.

Multiples of $6$: $\quad 6,12,18,24,30,36,{\color{blue}42},48, \ldots $

Multiples of $21$: $\quad 21, {\color{blue}42},63, \ldots$

We conclude that the least common multiple of $6$ and $21$ is ${\color{blue}{42}}.$

Therefore, the least common denominator of $\dfrac{1}{6}$ and $\dfrac{5}{21}$ is $42.$

The denominators of the given fractions are $10$ and $15.$ Thus to find the least common denominator, we find the least common multiple of $10$ and $15.$

Let's list the multiples of each number.

Multiples of $10$: $\quad 10, 20, {\color{blue}30}, ...$

Multiples of $15$: $\quad 15,{\color{blue}30},45, ...$

We conclude that the least common multiple of $10$ and $15$ is ${\color{blue}{30}}.$

So the least common denominator of $\dfrac 3 {10}$ and $\dfrac 2 {15}$ is $30.$

The denominators of the given fractions are $15$ and $25.$ To find the least common denominator, we find the least common multiple of $15$ and $25.$

Let's list the multiples of each number.

Multiples of $15$: $\quad 15, 30, 45, 60, {\color{blue}75}, \ldots $

Multiples of $25$: $\quad 25, 50, {\color{blue}75}, 100, \ldots$

We conclude that the least common multiple of $15$ and $25$ is ${\color{blue}{75}}.$

Therefore, the least common denominator of $\dfrac{1}{15}$ and $\dfrac{3}{25}$ is $75.$