3+5+7 = 15 total marbles
3 are red
so she has a 3/15 reduces to 1/5 probability of picking a red one
-12+3b-1=-5-b
Combine like terms to get:
3b-13=-5-b
Add 13 to both sides:
3b= 8-b
Add b to both sides:
4b=8
Divide both sides by 4:
4b/4=8/4
b=2
Final answer
b=2
