Question

A container is filled with 100 grams of bird feed that is 80% seed. Tom and Sally want to mix the 100 grams with bird feed that is 5% seed to get a mixture that is 40% seed. Tom wants to add 114 grams of the 5% seed, and Sally wants to add 115 grams of the 5% seed mix. What will be the percent of seed if Tom adds 114 grams?

Answer 1

Answer:

Initially, we had:

100 grams of a mixture that is 80% seed, (or 0.8 in decimal form)

This means that the quotient between the mass of seeds and the total mass of the mixture is equal to 0.8

Then the mass of seeds, x, can be calculated as:

x/100g = 0.8

x = 0.8*100g

x = 80g

We also have another mixture, that is 5% seed (or 0.05 in decimal form)

This means that in X grams of this mixture, there will be y grams of seeds, such that:

y/X = 0.05

y = 0.05*X grams

Now, we want to create a mixture that is 40% seeds (or 0.4 in decimal form).

Let's do this.

If we add X grams of the 5% mixture, the total mass will be:

M = 100g + X.

The total mass of seeds will be:

m = 80g + X*0.05

and we want that the quotient between the mass of seeds and the total mass to be exactly 0.4

m/M = 0.4

(80g + X*0.05)/(100g + X) = 0.4

Let's solve this for X.

(80g + X*0.05) =  0.4*(100g + X)

80g + X*0.05 = 40g + X*0.4

80g - 40g = X*0.4 - X*0.05

40g = X*0.35

40g/0.35 = X = 114.2g

If we want the mixture to be exactly 40% seeds, we need to add 114.2g of the 5% mixture. (We already can see that Tom is closer to the exact result than Sally)

If we add 114g, like Tom wants, we will have:

M = 100g + 114g = 214g

m = 80g + 0.05*114g = 85.7g

m/M = 85.7g/214g = 0.4005

This is a really good aproximation, if we write this in percentage form this would be:

0.4005*100% = 40.05%