of means multiplication
So
[tex] \frac{3}{4} \sf{of} \frac{16}{21} = \frac{3}{4} \times \frac{16}{21} [/tex]
And then simplify the numerator ad denominator
= [tex] \frac{3 \times 16}{4 \times 21} = \frac{\ 1 \times 16}{4 \times 7}[/tex]
And simplify the numerator and denominator again
= [tex]\frac{\ 1 \times 16}{4 \times 7} = \frac{\ 1 \times 4}{1 \times 7}[/tex]
And we are finally left with the answer which is [tex] \frac{4}{7} [/tex]
