Answer :

Sure, let's go through each part step by step.

### Part (i)
We need to find the product of two mixed numbers: [tex]\( 6 \frac{1}{8} \)[/tex] and [tex]\( 15 \frac{3}{4} \)[/tex].

1. Convert the mixed numbers to improper fractions:

- For [tex]\( 6 \frac{1}{8} \)[/tex]:
[tex]\[ 6 \frac{1}{8} = 6 + \frac{1}{8} = \frac{6 \times 8 + 1}{8} = \frac{49}{8} \][/tex]

- For [tex]\( 15 \frac{3}{4} \)[/tex]:
[tex]\[ 15 \frac{3}{4} = 15 + \frac{3}{4} = \frac{15 \times 4 + 3}{4} = \frac{63}{4} \][/tex]

2. Multiply the improper fractions:

[tex]\[ \text{Product} = \frac{49}{8} \times \frac{63}{4} = \frac{49 \times 63}{8 \times 4} = \frac{3087}{32} \][/tex]

3. Convert the result back to a mixed number (optional):

[tex]\[ \frac{3087}{32} = 96.46875 \][/tex]

Thus, [tex]\( 6 \frac{1}{8} \)[/tex] of [tex]\( 15 \frac{3}{4} \)[/tex] is [tex]\( 96.46875 \)[/tex].

### Part (ii)
We need to solve for [tex]\( x \)[/tex] if [tex]\( 8.4\% \)[/tex] of [tex]\( x \)[/tex] is 420.

1. Understand the percentage relationship:

[tex]\[ 8.4\% \text{ of } x = 420 \][/tex]

2. Convert the percentage to a decimal:

[tex]\[ 8.4\% = \frac{8.4}{100} = 0.084 \][/tex]

3. Set up the equation from the percentage problem:

[tex]\[ 0.084 \times x = 420 \][/tex]

4. Solve for [tex]\( x \)[/tex]:

[tex]\[ x = \frac{420}{0.084} = 5000 \][/tex]

Thus, [tex]\( x \)[/tex] is [tex]\( 5000 \)[/tex].

### Summary
1. [tex]\( 6 \frac{1}{8} \)[/tex] of [tex]\( 15 \frac{3}{4} \)[/tex] is [tex]\( 96.46875 \)[/tex].
2. The value of [tex]\( x \)[/tex] if [tex]\( 8.4\% \)[/tex] of [tex]\( x \)[/tex] is 420 is [tex]\( 5000 \)[/tex].