Question 27063
We must "translate" all this data into equations. Let's call A to the amount of flour, B to the amount of sugar, and C to the amount of salt. Now,

She needs twices as much flour as sugar: {{{A = 2B}}}

and twice as much sugar as salt: {{{B = 2C}}}

She plans to buy a total of 10.5 pounds of flour, sugar and salt combined: {{{A+B+C=10.5}}}

Given these three equations, we can solve the system. First substitute the second one into the first one:

{{{A = 2(2C) = 4C}}}

Then we substitute this result into the third one:

{{{4C+B+C=5C+B=10.5}}}

Finally, we subsitute again the second one into the one we've just found:

{{{5C+B=10.5}}}

{{{5C + 2C = 7C = 10.5}}}

{{{C = 10.5/7 = 1.5}}}

So shw will buy 1.5 pounds of salt. Since she will buy twice as much sugas as salt, she will buy 3 pounds of sugar. And clearly, she will be buy 6 pounds of flour.

Using the given prices, she will spend:

6*1.20 + 3*2.30+ 1.5*0.68 = 15.12

she'll spend $15.12