Probability
posted by Amponsah .
A bag contains some balls of which 1/4 are red. Forty more balls of which 5 are red are added. If 1/5 of all the balls are red, how many balls was there originary?

original number of balls  x
original number of red balls = x/4
number of red balls added = 5
total balls added = 45
new total = x+45
new red ball total = (x/4 + 5)
(1/5)(x + 45) = x/4 + 5
x/5 + 9 = x/4 + 5
times 20
4x + 180 = 5x + 100
x = 80
you had 80 balls
check:
you had 80 of which 20 were red
you added 45 of which 5 were red
new total 125
new number of red = 25
is 25 equal to 1/5 of 125 ? YES!!