Let B is the number of blue tickets and G is the number of green tickets We get 3B + 5G = 25 (1)
and B + G = 7 (2) from the (2) we have; B = 7 - G sustituting this value of B to the (1) we get
3(7-G)+5G=25 --->>> 21-3G+5G=25 -->> 2G = 4 so G=2 putting where G = 2 to (2) we get B+2=7
and B = 5