Question

To mix plaster for a dental model, 45 mL of water is used for 100g of plaster. How many mL of water should be used for 250g of plaster? Round your final answer to 1 decimal place if necessary.

Answer 1

Hello there. To solve this question, we'll have to remember some properties about values that varies directly.

Of course, when you mix plaster for a dental model, the more you use of plaster, the more you'll have to use water, so we say that the values varies directly.

In fact, the ratio between the amount of water, in mL, and the amount of plaster, in grams, will be a constant (called proportionality constant):

`(water)/(plaster)=k`

In this case, we can take this ratio to find how many water we'll need when the amount of plaster increases, as follows:

Say x is the amount of water we use for 250g of plaster. Knowing 45ml of water is used for 100g of plaster, we have:

`(45)/(100)=(x)/(250)`

Multiply both sides of the equation by a factor of 250

`x=250\cdot(45)/(100)=2.5\cdot45=112.5\text{ mL}`

This is how many water we need in this case.