Update tutorial.md - Assembly - Hands-0n: Quality control

Included a Q&A showing detailed calculation to find the  probability of error and the accuracy of a nucleotide base.

topics/sequence-analysis/tutorials/quality-control/tutorial.md

@@ -148,12 +148,19 @@ It means that the fragment named `@M00970` corresponds to the DNA sequence `GTGC
 >
 > 1. Which ASCII character corresponds to the worst Phred score for Illumina 1.8+?
 > 2. What is the Phred quality score of the 3rd nucleotide of the 1st sequence?
-> 3. What is the accuracy of this 3rd nucleotide?
+> 3. How to calculate the accuracy of the nucleotide base with the ASCII code /?
+> 4. What is the accuracy of this 3rd nucleotide?
 >
 > > <solution-title></solution-title>
 > > 1. The worst Phred score is the smallest one, so 0. For Illumina 1.8+, it corresponds to the `!` character.
 > > 2. The 3rd nucleotide of the 1st sequence has a ASCII character `G`, which correspond to a score of 38.
-> > 3. The corresponding nucleotide `G` has an accuracy of almost 99.99%
+> > 3. ASCII code for / is 47
+       Quality score = 47-33=14
+       Formula to find the probability of error
+       P = 10 -Q/10
+       Probability of error = 10-14/10 = 0.03981
+       Therefore Accuracy = 100 - 0.03981 = 99.96%
+> > 4. The corresponding nucleotide `G` has an accuracy of almost 99.99%
 > >
 > {: .solution }
 {: .question}