SOLUTION: A substance is 90% water. Some water evaporates, leaving a substance that is 80% water. What percent of the water evaporated? Omit the % sign, and express your answer as a decimal

Question 1174237: A substance is 90% water. Some water evaporates, leaving a substance that is 80% water. What percent of the water evaporated? Omit the % sign, and express your answer as a decimal to the nearest tenth.

Answer by greenestamps(13209) (Show Source): You can put this solution on YOUR website!

The original substance was 90% water, or 9/10 water; so the ratio of water to other material was 9:1.

After some water was evaporated so that the substance was 80% water, or 4/5 water, the ratio of water to other material was 4:1.

The amount of other material did not change. So the ratio of amounts of water in the substance after evaporation to the amount in the original substance was 4:9. That means 5/9 of the water in the original substance evaporated.

You can do the calculation and round the answer as directed.

It's a lot easier (to me, anyway) if we just put some numbers to the problem.

Suppose we start with 10 units of the substance; 9 units are water and 1 unit is something else.

When water is evaporated so that the substance is 80% water, that 1 unit of something else is now 20% of the substance. That means there are 5 units of the substance remaining, so 4 units are water.

We started with 9 units of water and finished with 4 units; 5 units evaporated.

Since 5 of the 9 units evaporated, the percent of water that evaporated is 5/9, converted to a percent.

Again I leave the simple calculations to you.

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