A money box contains only 10-cent and 20-cent coins. There are 28
coins with a total value of $3.80.
How many coins of each?

Number of 10 cents = 18

Number of  20 cents = 10

Step-by-step explanation:

Let number of 10 cents be = x

Let number 20 cents be = y

Total number of coins = x + y = 28  -------- ( 1 )

Total amount in the box = 0.10 x + 0.20y = 3.80 ---------- ( 2 )

Solve the equations to find x and y

( 1 ) => x + y = 28

         x = 28 - y

Substitute x in ( 2 )

( 2 ) => 0.10(28 - y) + 0.20y = 3.80

          2.80 - 0.10y + 0.20y = 3.80

            0.10 y = 3.80 - 2.80

            0.10 y = 1.00

                y = 10

Substitute y in ( 1 ) => x + y = 28

                                x + 10 = 28

                                x = 28 - 10

                                x = 18

a. is correct