The following system of equations represents the given statement; x = y+z y = 2z10x+15y+40z = 600They must sell 18 small pizzas, 12 medium pizzas and 6 small pizzas.Step-by-step explanation:Let, Small pizza = xMedium Pizza = yLarge Pizza = zCost of one small pizza = $10Cost of one medium pizza = $15Cost of one large pizza = $40According to given statement; They usually sells as many small pizzas as medium and large pizzas combined.x = y+z The number of medium pizzas sold is usually twice as many as large ones.y = 2zHow many of each pizza must they sell to get $600.10x+15y+40z = 600The following system of equations represents the given statement; x = y+z Eqn 1y = 2z Eqn 210x+15y+40z = 600 Eqn 3Putting value of x and y from Eqn 1 and 2 in Eqn 3;[tex]10(y+z)+15(2z)+40z=600\\[/tex]We know that y=2z from Eqn 2,[tex]10(2z+z)+30z+40z=600\\20z+10z+70z=600\\100z=600[/tex]Dividing both sides by 100;[tex]\frac{100z}{100}=\frac{600}{100}\\z=6[/tex]Putting z=6 in Eqn 2;[tex]y=2(6)\\y=12[/tex]Putting value of y and z in Eqn 1;[tex]x=12+6\\x=18[/tex]They must sell 18 small pizzas, 12 medium pizzas and 6 small pizzas.Keywords: linear equations, substitution methodLearn more about linear equations at:brainly.com/question/2869849brainly.com/question/2920401#LearnwithBrainly