Calculating grade for second year

EM20314

I'm hoping somebody can help me! I'm currently in my third year, but looking back at my second year grades and trying to work out the average correctly...

(the number in brackets is the credit value of each module)

63.40 (20)
57.50 (20)
53.00 (20)

70.00 (10)
77.80 (10)
68.00 (10)
62.00 (10)
48.00 (10)
68.00 (10)

My questions are:

The grade is calculated using my best 100 credits. My lowest grade is 48.00, but that's only a 10 credit module. So, do I just use the lowest grade out of my 20 credit modules instead (that would mean leaving 53 out only)?

Also, do I calculate the average by including the 20 credit modules twice?

Given the above assumption that 53 is omitted, I have calculated it in the following way:

63.40 + 63.40 + 57.50 + 57.50 + 70.00 + 77.80 + 68.00 + 62.00 + 48.00 +68.00 = 635.6
635.6/ 10 = 63.56
so, I got a 2:1 overall?!

If someone could clarify whether or not this is correct, it would be very much appreciated.
Thanks x