Pls explain how to do such type of ques

Pls explain how to do such type of ques . A m-RNA which codes for a protesn has 300 nucleotides. If the nucleotide at position 271 is cort:tb cg th.s mRNA 90 (3) 91 (2) too

Dear student,
Given: the protein has 300 nucleotides.
We know that three nucleotides together make one codon.
So, a protein with 300 nucleotide will have 300/3 = 100 codons.
Now, a nucleotide at position 271 is deleted i.e. the protein would have unaltered 270 nucleotides.
And 270 nucleotides make total 270/3 =90 codons.
So, if nucleotide at position 271 is deleted, 90 codons will be unaltered. 
Answer is option 1. 
Regards