A school has an average of 145 boys and girls. The average number of boys, girls and teachers is 100. How many teachers are there at the school?

Answer 1

If I'm reading the question correctly, (b+g)/2=145 and (b+g+t)/3=100; [definition of "average"]. From this, you can see that the sum of boys & girls is 290, while the sum of boys, girls & teachers is 300. I'm sure you can figure out the rest.