Question

the sum of two numbers is 15 less than twice the first number their difference is 5 less than twice the second number.  Find the number

Answer 1

$2n-15=5-2n$

$4n=20$

$\frac{4n}{4}=\frac{20}{4}$

$n=5$

The number is 5

Answer 2

X --- first number
y --- second number

x + y = 2x - 15
x - y = 2y - 5

x - 2x + y = -15
x - y - 2y = - 5

-x + y = -15
x - 3y = - 5
-----------------
-2y = -20  /:(-2)
y = 10

x - 3y = - 5   ⇒  x = - 5 + 3y = - 5 + 3 · 10 = - 5 + 30 = 25

Ans.:
x = 25
y = 10