Acoconut shy at the village fair offers three throws for $1, or seven throws for $2. in one hour, 187 throws are made and the attendent takes $57.
how many people bought three throws?

Total throws = 187

Let three throws = 3t

And

Seven throws = 7s

So

3t + 7s = 187 (1)

While total collected money = $57

Money for three throws = 1t

Money for seven throws = 2s

Hence

1t +2s = 57 (2)

Now solving both equations, so multiply second equation by 3 and subtract it from equation (1);

Equation (2) becomes 3t + 6s = 171

While subtracting it from equation (1) gives the final equation as below.

s = 16

Putting s = 16 in equation (2);

1t + (2x16) = 57

1t + 32 = 57

t = 25

Thus 25 people bought three throws and 16 people bought seven throws.

As for 25 people having three throws = $25

And

16 people having seven throws = 2x16 = $32

So total earned money = 25 + 32 = $57

Hence proved.