30% of the marbles are red at first and the rest are blue. When 15 marbles was added, the total percentage of red marbles increased by 20%. How many blue marbles are there in the end?
There's no way to tell.
If there are x marbles at first, then there are 0.30x reds.
Without knowing how many of the added 15 marbles are red, we are stuck.
We know that some of them must be red if the percent reds increased.
I guess you could try all values from 1 to 15 and see which produce integer solutions. That would give you
.30x+k = .50(x+15)
k = .20x+7.5
No value of k produces an integer value for x.
total marbles ---- x
red ---- .3x
blue --- .7x
after addition:
red added ---- r
blues added = 15-r
red : blue = 1:1 , when 20% is added to 30% ---> 50%
so reds = blues
.3x + r = .7x + 15-r
2r = .4x + 15
r = (.4x + 15)/2 = (4x + 150)/20 = (2x + 75)/10
now we know r ≤ 15 , and 2x has to end in 5 for the top to
be evenly divisible by 10. But 2x will always be even, thus cannot end in 5
Either something is missing or else the question is bogus.
e.g. were the 15 marbles added of one colour ??