How so you multiply ##\frac { 3} { 8} \times \frac { 16} { 21}##?

##2/7##

When multiplying fractions we look for any ##color(blue)"common factors"## on the numerators / denominators that we can ##color(blue)"cancel",## that is divide by.

Here 3 and 21 have a common factor of 3 and 8 and 16 have common factors of 2 , 4 and 8. We choose the highest common factor in this case 8.

##rArr3/8xx16/21=cancel(3)^1/cancel(8)^1xxcancel(16)^2/cancel(21)^7##

##=(1xx2)/(1xx7)=2/7larr" in simplest form"##

A fraction is in ##color(blue)"simplest form"## when no other factor but 1 will divide into the both the numerator and denominator.