An ordinary sponge is soaked with water so that this wet sponge is 1 kg. The mass of the water is 99% that of the wet sponge. The sponge is left outside for a certain period of time. When you come back, you find that the mass of the water is now 98% of the sponge. How much water is left in the sponge?

The system (sponge and water) weighs 1 Kg or 1000g.

The water weighs 99% of that, or 990g.

Therefore, the sponge weighs 1000 - 990 = 10g.

If in the new system the water is 98% of the system, then the weight of the sponge is 2% of the system.

If 10g = 2%, then the whole system weighs 10/0.02 = 500g

and the new weight of the water is 500 - 10 = **490g**.

i see almost everything …

but where did the 10g /0.02 come from?

i haven’t looked at a physics question in 6 years…

The weight of the sponge remains constant. Therefore, in the inital system, the sponge is 100% - 99% of the mass of the entire system. After evaporation, the sponge is 100% - 98%, or 2% of the entire system. 2% obviously equals .02

And this in an algebra question; it’s purely mathematical. Has nothing to do with physics.

This is not a physics question at all, it’s algebra.

The water weighs 98% of the sponge and the water together, therefore the sponge weighs 2% of the two. We already established that the sponge weighs 10g. So if 10g is 2% of the total weight, that is expressed as the formula

10g = 2% * total g =>

10g = 2/100 * total g =>

10g = 0.02 * total g =>

10g/0.02 = total g

ahhh i see the light…

of course, i haven’t taken algeo in 6 years either…

studying psychology tends to dull the math senses…

i should really brush up on that…