30 cans in total. 60% coke, 40% pepsi.
that means 18 cans is coke and 12 cans is pepsi.
After drinking some amount of cans of coke, now 25% is coke and 75% is pepsi.
the remaining amount has to be divisible by 4, so trying is probably faster than using equations.
if remaining 28, coke 16- no good
if remaining 24, coke 12- no good
remaining 20, coke 8- no good, but close
remaining 16, coke 4- that's our answer, and that means Noah drink 14 cans of coke.