Question

M + 5n; use m = 1, and n = 4

Answer 1

$m + 5n; \\ m = 1, \\ n = 4 \\ ======= \\ 1+5 \cdot4= \\ 1+20= \\ \boxed{21}$

Answer 2

If "m" = 1  and If "n" = 4 

Your problem becomes↓$1 + 5(4)$
                                                    
$5(4) = 20$

$1 + 20 = 21$

Therefore your answer is: $21$

Just do PEMDAS 

$Parentheses$
$Exponents$
$Multiplication$
$Division$
$Addition$
$Subtraction$

From the looks of it we have used the "addition " and "multiplication" method of the PEMDAS strategy