Question 918073
A dime is worth 2 nickels. 
 let no of dimes be x
  let no of nickles be y
  so as per the data mentioned in the question , x=2*y-10
total amount  = $2.75 
 so total amount should comprise of nickels and dimes
        x*dimes + y*nickels  = $2.75
           x*( 2 nickels)+ y *nickels = $2.75
           => nickles( 2x+y) = $ 2.75
                   but 20 nickels = 1 dollar
                nickles(2x+y) = 2.75*20 nickels
     cancelling nickles on both sides
                  2x+y = 55
       already we know that  x= 2y-10 
                    so 2(2y-10) +y= 55
                         4y-20+y=55
                           5y -20 =55
                            add 20 on both sides
          
                          5y-20+20=55+20
                5y=75
  divide with 5 on both sides
           5y/5  =75/5
        y= 15
    so x= 2y-10 
            x= 2*15-10 
                x= 30-10
                x= 20
so no of dimes   =20 
no of nickels   =15