A number is three less than four times another number. Their sum is 207, then the numbers are 

Let $x$ , $y$  be two numbers

Given $x=\left(4y-3\right)\cdots \cdots \cdots \left(1\right)$

             $x+y=207\cdots \cdots \left(2\right)$

Substitute equation$\left(1\right)$ in equation $\left(2\right)$

$x+y=207$

$\left(4y-3\right)+y=207$

$5y-3=207$

$5y=210$

$y=42$

Substitute $y=42$ in equation$\left(1\right)$

$x=\left(4y-3\right)$

$x=4y-3$

$=4\left(42\right)-3=168-3$

$x=165$