Suppose that the coding region of a gene contains 1,800 base pairs, with 570 in exon 1, 420 in exon 2, and 810 in exon 3 (not counting the stop codon). the protein translated from this gene will consist of amino acids

Each group of three nucleobases, when translated, will result in one amino acid. Considering that the coding region has 1800 base pairs, and only one side of the DNA strand is expressed, there will be only 1800 nucleobases that will take part on the gene expression and the protein translated will consist of 600 amino acids. It is only needed to divide 1800 by 3 (which is the number of nucleobases that will result in one amino acid). =600