Question 324538
Let the 3 colors be w,g,b for white, green and blue 


The number of blues was 7 less than the sum or the whites and the greens
w + g -7 = b
w + g - b = 7 ...... eq 1


The number of greens was 1 greater than the sum of the blues and the whites

g = 1 + b + w
b + w -g = -1 .........eq 2


three times as many greens as blues
3b = g ------eq 3


Substitute eq 3 in eq 1 
w + g - b = 7
w + 3b - b = 7
w + 2b = 7 -------eq 4

Substitute eq 3 in eq 2
b + w - g = -1 
b + w - 3b = -1 
w - 2b = -1 .........eq5

Add eq4 and eq 5 
w + 2b = 7
w - 2b = -1 

2w = 6
w = 3


Substitute w = 3 in eq 4 to get the value of b
w + 2b = 7
3 + 2b = 7
2b = 7-3 = 4
b = 2


Substitute w = 3, b = 2 in eq 1 to get the value of g

w + g - b = 7
3 + g - 2 = 7
1 + g = 7
g = 6


There are 3 whites, 2 blues and 6 greens