[tex]a = \frac{10 \times 4 + 15 \times 3 + 20 \times 5 + x \times 18}{4 + 3 + 5} = [/tex]
[tex] = 100[/tex]
[tex]a = \frac{40 + 45 + 100 + 18x}{12} = 100[/tex]
[tex]a = \frac{185 + 18x}{12} = 100[/tex]
[tex]185 + 18x = 1200[/tex]
[tex]18x = 1200 - 185[/tex]
[tex]18x = 1015[/tex]
[tex]x = 1015 \div 18[/tex]
[tex]x = 56.38[/tex]
[tex]a = 56.38[/tex]
