the question can be changed into: what percent of the time will you choose yellow or blue cards?
to solve, you would first add the three numbers for the total: 60. Then you would subtract the amount of green cards from the total to get the amount of blue and yellow: 60-18=42. 42/60 of the time, you will not choose green. if you simplify, then 7/10(70%) of the time, you will not choose green. :)
