A (simple?) problem from a non-mathematical person!

Hi,

I'm at university, I wanted to find a formula that would calculate what average I need to achieve for the rest of the year.

At our university we do 120 credits every year. The second year is worth 25% of the total mark, the third year is worth 75%.

So, say (for example) I got an average of 55% in my second year. How would I work out what I needed to average in my final year to attain an overall average of 60%?

I know this might look simple to you - but I haven't studied maths for the last 8 years and I just can't fugure it out!