Answer :
x - sum of 20 numbers
[tex]\frac{x}{20}=18\\ x=360\\\\ \frac{x+3\cdot10}{20}=\frac{x+30}{20}\\ \frac{360+30}{20}=\frac{390}{20}=19.5[/tex]
[tex]\frac{x}{20}=18\\ x=360\\\\ \frac{x+3\cdot10}{20}=\frac{x+30}{20}\\ \frac{360+30}{20}=\frac{390}{20}=19.5[/tex]