Answer: he had twenty three $10 coins and twenty four $20 coins.
Step-by-step explanation:
Let x represent the number of $10 coins that he has.
Let y represent the number of $20 coins that he has.
Dave has a collection of 47 coins. This means that
x + y = 47
Some are $10 coins and the rest are $20 coins. If the face value is $710, it means that
10x + 20y = 710- - - - - - - - - -1
Substituting x = 47 - y into equation 1, it becomes
10(47 - y) + 20y = 710
470 - 10y + 20y = 710
- 10y + 20y = 710 - 470
10y = 240
y = 240/10
y = 24
x = 47 - y = 47 - 24
x = 23
