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Ujarak has 
100mL of a strong toothpaste with a 
1.1% concentration of sodium fluoride. They have a weaker toothpaste with a 
0.3% concentration of sodium fluoride.
What volume, in milliliters, of the weaker toothpaste would Ujarak need to add to the strong toothpaste to create a blend with a 
0.8% concentration of sodium fluoride?

mL

Ujarak has 100 mL 100 \mathrm{~mL} of a strong toothpaste with a 1.1% 1.1 \% concentration of sodium fluoride. They have a weaker toothpaste with a 0.3% 0.3 \% concentration of sodium fluoride.\newlineWhat volume, in milliliters, of the weaker toothpaste would Ujarak need to add to the strong toothpaste to create a blend with a 0.8% 0.8 \% concentration of sodium fluoride?\newlinemL \mathrm{mL}

Full solution

Q. Ujarak has 100 mL 100 \mathrm{~mL} of a strong toothpaste with a 1.1% 1.1 \% concentration of sodium fluoride. They have a weaker toothpaste with a 0.3% 0.3 \% concentration of sodium fluoride.\newlineWhat volume, in milliliters, of the weaker toothpaste would Ujarak need to add to the strong toothpaste to create a blend with a 0.8% 0.8 \% concentration of sodium fluoride?\newlinemL \mathrm{mL}
  1. Set up initial amount and concentration: Set up the initial amount and concentration of the strong toothpaste.\newline100mL100\,\text{mL} of toothpaste with a 1.1%1.1\% concentration of sodium fluoride.\newlineFirst expression: 100mL×1.1%=100mL×(1.1100)=100mL×0.011100\,\text{mL} \times 1.1\% = 100\,\text{mL} \times \left(\frac{1.1}{100}\right) = 100\,\text{mL} \times 0.011
  2. Set up variable for weaker toothpaste: Set up the variable for the amount of weaker toothpaste to be added and its concentration.\newlineLet xx be the volume in milliliters of the 0.3%0.3\% sodium fluoride toothpaste to be added.\newlineSecond expression: xx mL ×0.3%=x\times 0.3\% = x mL ×(0.3/100)=x\times (0.3/100) = x mL \times \(0.003003
  3. Set up expression for total volume and desired concentration: Set up the expression for the total volume and desired concentration of the final blend.\newlineThe total volume will be 100mL+xmL100\,\text{mL} + x\,\text{mL}, and the desired concentration is 0.8%0.8\%.\newlineFinal expression: $(\(100\)\,\text{mL} + x\,\text{mL}) \times \(0\).\(8\)\% = (\(100\)\,\text{mL} + x\,\text{mL}) \times \left(\frac{\(0\).\(8\)}{\(100\)}\right) = (\(100\)\,\text{mL} + x\,\text{mL}) \times \(0\).\(008\)
  4. Create equation based on amounts and desired concentration: Create an equation based on the amount of sodium fluoride in the initial strong toothpaste, the amount to be added from the weaker toothpaste, and the desired final concentration.\(\newline\)Equation: \((100\,\text{mL} \times 0.011) + (x\,\text{mL} \times 0.003) = (100\,\text{mL} + x\,\text{mL}) \times 0.008\)
  5. Distribute concentrations and set up equation: Distribute the concentrations and set up the equation.\(\newline\)\(1.1\)mL + \(0.003x\) mL = \(0.8\)mL + \(0.008x\) mL
  6. Rearrange equation to isolate variable x: Rearrange the equation to isolate the variable x on one side.\(\newline\)\(1.1\,\text{mL} + 0.003x\,\text{mL} - 0.008x\,\text{mL} = 0.8\,\text{mL}\)\(\newline\)\(1.1\,\text{mL} - 0.005x\,\text{mL} = 0.8\,\text{mL}\)
  7. Solve for volume of weaker toothpaste: Solve for \(x\), which represents the volume of \(0.3\%\) sodium fluoride toothpaste needed.\(\newline\)\(1.1\)mL \(- 0.005x\) mL \(= 0.8\)mL\(\newline\)\(0.005x\) mL \(= 1.1\)mL \(- 0.8\)mL\(\newline\)\(0.005x\) mL \(= 0.3\)mL\(\newline\)\(0.3\%\)\(0\)\(\newline\)\(0.3\%\)\(1\)mL

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