x - the numbers of hits
y - the number of times at bat
He has a 0.300 batting average.
[tex]\frac{x}{y}=0.3 \\
x=0.3y[/tex]
If he gets a total of 10 more hits the next 10 times at bat, his batting average will be 0.400.
[tex]\frac{x+10}{y+10}=0.4 \\
x+10=0.4(y+10) \\
x+10=0.4y+4 \\
x=0.4y+4-10 \\
x=0.4y-6[/tex]
[tex]x=x \\
0.3y=0.4y-6 \\
0.3y-0.4y=-6 \\
-0.1y=-6 \\
y=-6 \times (-10) \\
y=60 \\ \\
x=0.3y=0.3 \times 60=18[/tex]
He had 18 hits.