Question 843480
Set b = no of boys , g = no of girls
Set m = total of marks for the class
g = 30 - b
m = 63*30
m = 1890
m = 65g + 60b
1890 = 65g + 60b
substitute 30 - b for g in above
1890 = 65(30 - b) + 60b
1890 = 1950 -65b + 60b
1890 = 1950 -5b
add -1950 to each side
-60 = -5b
divide each side by -5
12 = b
Since g = 30 - b ,
g = 30 - 12
g = 18
Substitute into 1890 = 65g + 60b to check
1890 =? 65(18) + 60(12)
1890 =? 1170 + 720
1890 = 1890