I'm struggling with my maths! can someone !

1)
2400kg-15% water=
2400-360=2040kg other ingredients

1+2+5=8
->
1/8=cement, 2/8 sand, 5/8 stone

2040*(1/8)=2040/8=255kg cement
2040*(2/8)=2040/4=510kg sand
2040*(5/8)=(2040/8)*5=255*5=1275kg stone

2)

A=2/15 + 1/3 =2/15 + 5/15=7/15
B=6/7 - 1/3=18/21 - 7/21=11/21

15=3*5
21=3*7
-> least common multiple of 15&21=3*5*7=105
A=7/15=(7*7)/105=49/105
B=11/21=(11*5)/105=55/105

now we can compare them, but we still don't know how they relate to 1/2

-> least common multiple of 105&2=210
A=49/105=98/210
B=55/105=110/210
1/2=105/210

now you can see the difference between 1/2 and A is 7/210 and between 1/2 and B is 5/210, so B is closer to 1/2 than A

3)
a)insert n=2 and calculate the value:
3*(2)^2-1=
3*4-1=
12-1=11

b)insert n=5 and calculate the value:
3*(5)^2-1=
3*25-1=
75-1=74