hi,

first, the 140 fifty-cent coins = 70 one-dollar coins.

[i use ..... for spacing; 'u' means unit, and 'p' means part]

Now, tabulate the no. of coins as:

...................... $1.................. 50 cts

at first:............2u...................5u

change:........(+70)..............(-140)

in end:.............5p..................2p

So,

2u + 70 = 5p

x2: 4u + 140 = 10p

5u - 140 = 2p

x5: 25u - 700 = 10p

thus, 4u + 140 = 25u - 700

that is, 21u = 840

and, 1u = 40

there were 5u = 200 fifty-cent coins at first.

So, value of fifty-cent coins at first = 200 x $0.50 = $100

Hope this helps,

cheerio.