Explanation:

Given that the ratio of the number of boys in the first and the second standards is 2 : 3 and the ratio
The number of boys in the second and third standards is 4 :5
Now, we calculate a common ratio for all the three standards 2 : 3 and 4 : 5 will be 2×4:3×4 = 8 : 12 and 4×3 : 5×3 = 12 : 15
Therefore, the common ratio for all the three standards 8 : 12 : 15
Sum of the ratio pats = 8 + 12+ 15 = 35
Numbers of the boys in the first standard = (8/35)×350 = 80
Number of boys in third standard = (15/35)×350=150
Total number of boys in the both standards = 80+150= 230