Question

Sally has twice as many dimes as nickels. The total value is $3.50 how many nickels and dimes does she have?

Answer 1

2n=d
nickel is 0,05 dollars     dime is 0.1 dollars
0.05(n)+0.1(2n)=3.5
0.25n=3.5
n=14
d=2(14)=28
hope u can understand

Answer 2

D=2n
.05n+.1d=3.5
I multiplied the nickels and dimes by these values because that is their actual monetary value 
.05n=3.5-.1d
multiply everything by 20
n=70-2d
d/2=70-2d
.5d+2d=70 
2.5d=70
d=28 
n=14 
so there are 28 dimes and 14 nickels 
checking this answer 
28 dimes = $2.8
14 nickels = .70
2.8+.7=3.5