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

Answer 1

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.