Answer:

' Ask the user for three numbers
' Add them up, print the sum.
'
PRINT "What is the first number"
INPUT NUMBER1
'
PRINT "What is the second number"
INPUT NUMBER2
'
PRINT "What is the third number"
INPUT NUMBER3
'
LET   SUM = NUMBER1 + NUMBER2 + NUMBER3 
PRINT "The total is:", SUM
'
END

(You might have thought of a better answer.)

Slightly Changed Program

The above program works, and would be OK if all you wanted to do was add up three numbers. But is is somewhat awkward. Here is a somewhat different version:

' Ask the user for numbers.
' Add them up, print the sum.
'
LET  SUM = 0                 ' Clear the sum
'
PRINT "Enter a number"       ' First number
INPUT NUMBER                 ' User enters first number
LET   SUM = SUM + NUMBER     ' increase the SUM
'
PRINT "Enter a number"       ' Second number
INPUT NUMBER
LET   SUM = SUM + NUMBER
'
PRINT "Enter a number"       ' Third number
INPUT NUMBER
LET   SUM = SUM + NUMBER
'
PRINT "The total is:", SUM   ' At this point, SUM is the total of all numbers
'
END

To understand this program, review how the LET statement works:

A LET Statement

Finds memory for each NEW variable in the statement.
Does the calculation on the RIGHT of the equal sign.
Replaces the contents of the variable to the LEFT of the equal sign with the result of the calculation.

QUESTION 9:

Say that the first four statements of the program are executed and that that the user enters "12".

' Ask the user for numbers.
' Add them up, print the sum.
'
LET  SUM = 0                 ' Clear the sum
'
PRINT "Enter a number"       ' First number
INPUT NUMBER                 ' User enters first number
LET   SUM = SUM + NUMBER     ' increase the SUM

  . . . .

What value will be in SUM at this point?