FIND_SUM:  //the label where it starts may change syntax 
XOR DX,DX  // makes dx = 0 
XOR AH,AH  // makes AH = 0 
PUSH CX	   // save how many bytes (in your case 5 should be here) 
CLD		   // clear direction flag moves SI to the next byte each load 
FIND_SUM_LOOP1: 
LODSB	   // moves first byte into AL 
ADD DX,AX  // used DX,AX instead of DL,AL so there is no issue of overflow 
DEC CL     // decrement counter 
JNZ FIND_SUM_LOOP1 //if its no zero do the loop 
POP	CX    // reached the end, at this point DX has the total sum 
MOV AX,DX // copies the sum into ax (to do the average) 
DIV CL    // now CL has the average rounded down. 
RET		  // returns from where it was called. 
 
to use this lets suppose you have a variable name so you need to read the efective address of such variable suppose you name it stnumber5 
 
LEA SI,stnumber5 
MOV CX,5 
CALL FIND_SUM