The numbers in the equations are of the form.
ab ÷ c = (a+b),(ab ÷ c)
So in case of the first equation we have;
16 ÷ 4 = (1 + 6),(16 ÷ 4) = 74
Similarly we can try with the other 2 equations;
21 ÷ 7 = (2 + 1),(21 ÷7) = 33
&
81 ÷ 9 = (8 + 1),(81 ÷ 9) = 99
Hence we can find the missing number as;
55 ÷ 5 = (5 + 5),(55 ÷5) = 1011
Answer: 1011