Good question

Question

A substance is 99% water .some water evaporates leaving a substance that is 98%water.how much of the water evaporated?

Answer 1

x represents the amount of water evaporated

w represents the original amount

0.99w - x = 0.98(w - x)

0.99w - x = 0.98w - 0.98x 

0.01w - x = - 0.98x

0.01w = 0.02x

0.5w = x

 

Half (50%) of the water evaporated.

Answer 2

a substance is 99% water

some water evaporated leaving a substance that is 98% water

how much water evaporated ?

assume the substance weighs 100 lbs, 1%, or 1 pound is solid and 99% is water

some water evaporated and the substance is now 98% water but still 1 pound solid

the 1 pound solid is now 2% of the substance and the rest of the substance (98%) is water

2% of what number is 1 ?

2%*x=1

0.02x=1

x=1/0.02

x=100/2

x=50 pounds

1 pound makes up the solid and the rest is water (49% water)

49/50=98/100=98% water

50% out of the 99% of water evaporated (50 pounds of water evaporated)

a simpler way to do the problem is...

98%=98/100=49/50 where 49% is water and 1% is solid so 99%-49%=50%

 

Answer 3

How can this question be answered without knowing how much of the substance was present to begin with.  For example, the answer would be different if you started with 10 gallons vs. starting with 1gallon.  Unless they are asking "what % of the water evaporated"?  If so, the question should be re-written as such.