sally has seven times as many dimes as nickles their value is $3.00 how many nickles does she have

Let the number of nickles = N

Then....the number of dimes = 7N

And

No. of nickles * their value each  + No. of dimes * their value each =  total value

So we have

N(5) + 7N(10 )  = 300        simplify

5N + 70N     =  300

75N =  300        divide both sides by 75

4 = N     ⇒    [ 4 nickles ]

how did you get the 5 and 10