To write the answer as a percent, first we can show his score in a decimal.
We can do this by dividing the number of question he answered correctly by the total number of question:
13÷20= 0.65
To express decimals as a percent always multiply the decimal by 100 or move the decimal point 2 times to the right (moving it once to the right would be equivalent to multiplying by ten and once to the left is equivalent to dividing by 10)
0.65*100 = 65
Therefore, Sean got a 65% on his spelling test
~I hope this helps!~