Lisa took a full coffee mug and drank 1/5 of its contents. When Lisa wasn't paying attention, Shelly refilled the mug by adding water to the remaining coffee and then stirring. Lisa drank 1/4 of this mixture. When she looked away again, Shelly refilled the mug with more water and stirred, and Lisa drank 1/3 of this mixture. When Lisa looked away a third time, Shelly refilled the mug with more water. What percentage of this final mixture was water?

Question

cwrw238 · Answer

just for arguments sake suppose capacity of cup = 100 ml (tiny cup!)
after first refill amount water = 100/5  = 20 mls
after 1/4 taken 15 mls remain
second refill  - 25 mls added - total 40 mls
lisa drank 1/3 so  40 - 13 1/3 = 26 2/3 mls left
add 33 1/3 for last refill = 26 2/3 + 33 1/3 =  60 mls
60 %

anonymous · Answer

im stuck after 15. how do you get 40 and the 33 1/3

cwrw238 · Answer

well 1/4 of cup was drunk so 1/4 of 100 mls  = 25 mls was added .
15 mls of water was there already giving 15+25 = 40 mls water

anonymous · Answer

oh i see now u put -25 not +25

cwrw238 · Answer

oh i see i've got a habit of using -  it meant a hyphen not a minus sign

cwrw238 · Answer

it can cause confusion

anonymous · Answer

oh okay well thanks anyway

cwrw238 · Answer

yw