My son had the following word problem and I don't know where to begin:The ratio in the school gym of boys to girls is 4:3. 160 boys left the gym and the ratio became 4:5. How many girls are in the gym?

Answer 1

4/3 = b/g so b = 4 g/3

4/5 = (b-160)/g
or
4 g = 5 (4g/3 - 160)

4 g = 20 g/3 - 800
12 g = 20 g - 2400
so
8 g = 2400
g = 300