Step-by-step explanation:
missing where ?
I guess you mean that there is one element missing between 51 and 33 (based on the readable answer options).
if that is true, then the missing element is 42.
the difference between 60 and 51 is 9.
the difference between 51 and 33 is 18.
if there is an engender missing in between 51 and 33, then it is logical to put it in the middle cutting the interval of 18 in half to 2 times 9. which is also supported by the first interval between 60 and 51 (9).
so, it would be 51 -9 = 42