Help I'm really stuck here

Ruth has a beaker containing a solution of 800 mL of acid and 200 mL of water. She thinks the solution is a little strong, so she drains 150 mL from the beaker, adds 150 mL of water, and stirs the solution. Ruth thinks the solution is still too strong, so again she drains 150 mL from the beaker, and adds 150 mL of water. How many mL of water are now in the beaker?

maximum Aug 25, 2023

#1**+1 **

She starts with 1000ml of solution

Initially the ratio of water to acid is 200 : 800 = 1 : 4

When she pours off 150 ml of solution the first time she now has 850 ml of solution

The ml of water now = 1 /( 1 + 4) *(850) = 1/5 * 750 = 170ml

And the ml of acid = 850 - 170 = 680 ml

Note that the ratio of water to acid is still 170 : 680 = 1 : 4

When she adds 150 ml of water the first time she now has 1000ml again

But now she has 170 + 150 = 320 ml of water (the acid amt hasn't changed)

The new ratio of water to acid is 320 : 680 = 8 : 17

The second time she pours off 150ml the ratio of water to acid has not changed

She now has 850 ml of solution

The ml of water is now 8 / ( 8 + 17) * 850 = 272

The ml of acid is 850 - 272 = 578 ml

Finally ....adding back 150 ml of water gives us 1000 ml again

She now has 272 + 150 = 422 ml of water

And she still has 578 ml of acid

So she ends up with 422 ml of water and 578 ml of acid

The final ratio of water to acid is 422 : 578 = 211 : 289

CPhill Aug 25, 2023