Question 767570
You need to determine what 15 is 20% of, because he lost 20% of the marks.  
That will give you the total marks the test was worth.


a = pb {percent formula for any basic percent problem}
a is the part, p is the percent (as a decimal), b is the whole


15 is 20% of what number?
a = pb
15 = 0.2b {substituted values into percent formula, changing 20% to decimal form}
b = 75


The test was worth 75 marks.  He lost 15.
He got 60.
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