3. The materials to make 25kg of an alloy of copper and zinc cost $124. If the copper costs $6.40/kg and the zinc cost $2.80/kg, find the composition of alloy.

The composition is;

10 kg of copper and 15 kg of zinc

Step-by-step explanation:

let the weight of zinc be z and that of copper be c

The addition of both will give 25 kg

thus;

z + c = 25 ••••••(i)

secondly;

6.40(c) + 2.80(z) = 124

6.40c + 2.80z = 124 ••••••(ii)

from the first equation;

z = 25-c

substitute this into the second equation;

6.4c + 2.8(25-c) = 124

6.4c + 70 - 2.8c = 124

6.4c-2.8c = 124-70

3.6c = 54

c = 54/3.6

c = 15

Recall;

z = 25-c

z = 25-15

c = 10

answer set (b) 0.5, 3/4 80% is in order from least to greatest.  

step-by-step explanation:

in order to compare rational numbers to each other, it is much easier if they are in the same form.   typically, the best form in which to evaluate rational numbers is in decimals.   in order to convert a percent to a decimal, you would divide the given percentage by 100.   you can also accomplish the conversion by moving the decimal point over two places to the left.   for example, the decimal value of 80% is 80/100, or 0.8.   in order to convert a fraction to a decimal, you need to divide the numerator by the denominator.   for example, 3/4 is 0.75.   so, for choice (b) 0.5 3/4 (0.75), 80% (0.8) is the only choice in which the numbers are in order from least to greatest.