Suppose you have 20 coins that total $4.00. some coins are nickels and some are quarters. which of the following pairs of equations would you use to find out how many of each coin you have?
a. n+q=45n+25q=20
b. n+q=20n+q=4.00
c. n+q=4.00200.5n+0.25q=4.00
d. n+q=205n+25q=400

To find this...we need to see how much the total AMOUNT is. 
N is for  nickels and Q is for Quarters.

n+q=20 would be our first equation. 

We could see which choices have this as the first equation and cross the rest of the others out.

B and D have this...so we are left with those two choices.

n+q=20

Since we do not know how many nickels and quarters there are, we already know how much each is worth. 

Since we have that info, we can write a second equation.

 5n+25q=4.00

Now their is only one  answer that states this. 

Our final answer is that (D) n+q=20  5n+25q=400 is our answer.