How do i calculate degree classifications?

I study mathematics bsc and ive already got my second year results, however I wonder what I need to get in my final year to get a 1st overall.

The way the uni classifies degress is as follows:
-the best 60 credits from second year (weighted 25%)
-and 120 from final year (weighted 75%)

So the results I got for the best 60 are:
81%, (15 cred module)
73%, (15 cred module)
71% (15 cred module)
85% (15 cred module)

my question is, how do I calculate what i need to achieve per module in the final year to get an overall 180 credits which result in first class?

And would it be possible for someone to do this for me (showing the working)?

thank you

Bit worrying you're doing a maths degree and can't calculate this, but anyway:

81+73+71+75=300
300/4=75
75*0.25=18.75

18.75+0.75x=70
x=(70-18.75)/(0.75)=68.3%

Assuming a first is 70. If not, change the bold 70 to whatever a first class mark is.

So you need 68.3% average from your best 120 credits.