Let the number be x.
double it and add 5
⇒ 2x + 5
The result is 1 less than the number she thought of
⇒ x - 1
Construct the equation to solve for x:
2x + 5 = x - 1
x + 5 = -1 // Take away x from both sides
x = -6 // Take away 6 from both sides
----------------------------------------------------------------------
Answer : The number Tui first thought of is -6.
----------------------------------------------------------------------
