Dave Bowers collects U. S. gold coins. He has a collection of 49 coins. Some are​ $10 coins, and the rest are​ $20 coins. If the face value of the coins is ​$690​, how many of each denomination does he​ have? How many $10 coins does he have?
How many $20 coins does he have?

He has twenty nine $10 coins and twenty $20 coins

Let x be number of $10 coins and y be number of $20 coins
x + y = 49. Number of coins
10x + 20y = 690. Value of coins

Multiply the first equation by 10
10x + 10y = 490

Subtract from the second equation
10y = 200
y = 20

Substitute into equation 1
x + 20 = 49
x = 29

What mathematical statement is missing ? ? the answer is b

/-/////////looks tough ‍♂️