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In Emily's first spelling test of this new term she scored one mark out of five, so she decided to work really hard to improve her scores. Assuming that she succeeds on scoring full marks in all her tests after the first, how many more tests does she need to take to increase her average to more than $80$%.
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.