Answer:
$42.88
Step-by-step explanation:
Let's create a proportion using the following setup: 
cost/pounds=cost/pounds
We know that it costs $20.42 for a 10 pound turkey. 
$20.42/10 pounds= cost/pounds 
We don't know how much a 21 pound turkey costs, so we can say that it costs $x for a 21 pound turkey. 
$20.42/ 10 pounds= $x/ 21 pounds 
20.42/10=x/21 
We want to find x, by getting x by itself. 
x is being divided by 21. The inverse of division is multiplication. Multiply both sides by 21. 
21*(20.42/10)=(x/21)*21 
21* 20.42/10=x 
21*2.042=x 
42.882=x 
Round to the nearest cent, or hundredth. 
42.88=x 
x= $42.88 
A 21 pound turkey costs $42.88