If 3 pieces of candy cost $2.10, how much would four pieces cost?

First of all, all you have to do is compare the two statements
3 pieces=$2.10
4 pieces= Y
Y is the unknown value
Next, cross and multiply. That will be;
4 x 2.10=3 x Y
8.4=3Y
Divide both sides by 3
Y=8.4/3
Y=2.8
It will cost $2.80 or $2.8
Hope that helped. Have a nice day