Question

[Warm-up for previous challenge] Mark only has $1 coins and $2 coins in his wallet (poor guy). He has 10 coins and the total calue of the coins is $16. Find the number of $1 and $2 coins.

Answer 1

Let the number of $1 coins be x.
Number of $2 coins = 10 - x.
x + 2(10 - x) = 16
x + 20 - 2x = 16
-x = -4
x = 4
10 - x = 6

Answer 2

hope u understand this
the total sum of coins in mark's wallet is 10.
But in mark's wallet he has an unknown number of $1 and 2$ coins. And the sum of the total coins in his wallet is $16.
So lets take the unknown number of 1$ as x.
And the unknown number of 2$ as y.

Since the total number of coins in his wallet is 10
hence the sum of the unknown number of coins is also 10
that is, x+y = 10 -------- equation. 1
So then the total value of coins is
for simplicity sake i removed the $ sign
x(1) + (2)y = 16. ------ equation 2

if x + y = 10
then, x = 10 - y ---- equation 3
so substitute in equation 2,

(10 - y) 1 + 2 (y) = 16
10 -y + 2 = 16
12 - y = 16
y = 16 - 12
y = 4

from eqn 1
x + y = 10
substitute the value for x
x + 4 = 10
x = 6
so mark has six 2$ coins and four 1$