Let n be the number of blue ballons
Number of balloons Sheela bought = 60
Number of yellow colored balloons = 15
Number of red colored balloons = 28
Number of blue balloons, n = Total number of balloons - number of yellow colored balloons - number of red colored balloons
n = 60 - 15 - 28 = 17
So, the number of blue colored balloons she bought, n = 17.