An exam worth 145 points contains 50 questions. Some of the questions are worth two points and some are worth five points. How many two point questions are on the test? How many five point questions are on the test?

a*5+b*2=145     
a    +b   =50       | * -2

5a   +2b=145
-2a  -2b = -100
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5a-2a    = 145-100
3a=  45
a=45:3
a=15 questions are worth 5 p
b=50-15=35 questions worth 2 p