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

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

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