The mean is the sum of all values divided by the number of values.
So, the mean of the sales is:
[tex]\frac{54+45+33+39+48+40+21}{7}=\frac{280}{7}=40[/tex]
In order for the mean to be 45, x more ice creams have to be sell:
[tex]\begin{gathered} \frac{54,45,33,39,48,40,21+x}{7}=45 \\ \frac{280+x}{7}=45 \end{gathered}[/tex]
Multiplying both sides by 8:
[tex]\begin{gathered} \frac{280+x}{7}*7=45*7 \\ 280+x=315 \\ \end{gathered}[/tex]
Subtracting 280 from both sides:
[tex]\begin{gathered} 280+x-280=315-280 \\ x=35 \end{gathered}[/tex]
Answer: 35.