Given that your grade for your computing is based on 5 coursework that weigh differently, and you know the results of 4: 80, 75, 50 and 90 which weighs 10%, 20%, 45% and 5%. What grade do you need in your last coursework to achieve at least a B (70%)?

You know the following: 
A) 80, 10%
B) 75, 20%
C) 50, 45%
D) 90, 5%
E) X, (100-10-20-45-5)% = 20%
Therefore, create a formula with the facts given and the mean marks must be at least 70.
Note that 10% is equivalent to 0.10 and X is the mark you will need to get to achieve a B.
=> 80 x 0.10 + 75 x 0.20 + 50 x 0.45 + 90 x 0.05 + X x 0.20 >= 70
Rearranging the equation you get:
X x 0.20 >= 70 x 5 - (80 x 0.10 + 75 x 0.20 + 50 x 0.45 + 90 x 0.05)
0.20X >= 20
X >= 100
Therefore, the student will need to achieve exactly 100 (since the maximum mark allowed in the coursework is 100) in his final coursework to obtain a B which is highly unlikely to happen, so the student should have worked harder on his 3rd coursework which weighs 45%.