Font know hope you figure it out
Eliminate the fractions by multiplying by least common multiple.
4,9,12 all go into 36 evenly. Multiply entire equation by 36.
36/4 = 9
36/9 = 4
36/12 = 3
New equation looks like:
6 cookies for every 4 plates....6/4 = 1.5 cookies per plate
2 brownies for 4 plates.....2/4 = 0.5 brownies per plate
for 10 plates...
1.5(10) = 15 cookies <==
0.5(10) = 5 brownies <==
X = (1/2)y + 6
x + y = 21
Substitute (1/2)y + 6 as x into the equation x + y = 21 since x = (1/2)y + 6.
[(1/2)y + 6] + y = 21
0.5y + 6 + y = 21
1.5y = 15
y = 10
Same thing, now substitute 10 as y in the other equation.
x + y = 21
x + (10) = 21
x = 11