Answer :

16/21 divided by 4/9 is equivalent to 16/21 x 9/4 = 144/84 which reduces to 12/7. So 12/7 is your answer.
when solving a problem in division you make it a multiplication problem by INVERTING the divisor 16/21 X 9/4 You can cross cancel or just multiply the numerators/denominators and reduce 4/7 X 3/1= 12/7