How do you calculate a test value that only effects your grade by a certain percentage?

Question

My teacher has talked about how a certain test will only effect twenty percent of your final mark, how do you calculate this?

Say my current total mark is 95%, I got a test score of 70%, but the test only accounted for 20% of my final mark.

Answer 1

Heres how I found my answer.

The test makes up twenty percent of my mark and I got seventy percent then I write

70 * .2 = 14

My current mark(95%) takes up the leftover eighty percent so

95 * .8 = 76

Then, I add them together to get my final mark of 90