Math
posted by Nicole .
Susan has some $2coins and $5coins. If there are 18 coins and the total amount of these coins is not less than $75, find the minimum number of $5coins.

Let x = 2$ coins
Let y = 5$ coins
First equation is for the total amount of coins:
x + y = 18
Second Equation is for total amount in dollars in total from the coins:
2x + 5y = $75
Lets isolate for y
x = 18  y
Sub into second equation
2(18  y) + 5y = 75
36  2y + 5y = 75
3y = 39
y = 13
Therefore, you must have at least 13 $5 coins.