$\text{a. (4x + 5) : 3 - 121 : 11 = 4}$
$\text{(4x + 5) : 3 - 11 = 4}$
$\text{(4x + 5) : 3 = 4 + 11}$
$\text{4x + 5) : 3 = 15}$
$\text{4x + 5 = 15 × 3}$
$\text{4x + 5 = 45}$
$\text{4x = 45 - 5}$
$\text{4x = 40}$
$\text{⇒ x = 10}$
$\text{1 + 3 + 5 + ... + x = 1600}$
$\text{($\dfrac{x - 1}{2}$ + 1)² = 1600}$
$\text{$\dfrac{x - 1}{2}$ + 1 = 40}$
$\text{$\dfrac{x - 1}{2}$ = 40 - 1}$
$\text{$\dfrac{x - 1}{2}$ = 39}$
$\text{x - 1 = 39 × 2}$
$\text{x - 1 = 78}$
$\text{⇒ x = 79}$
