21 is 70% of what number

if the number is X, then 70 % of the number X has a value of 21.
then we can write it as an equation as follows
X * 70 % = 21
since 70% is equivalent to 0.7
                                                             
X * 0.7 = 21
                                                                     
X = 21 / 0.7
                                                                         
X = 30
                                                                     
therefore number is 30

The answer would be d i think but i’m for sure it is not a and c

The shelves will hold 270 books in total