Final answer:
A conditional statement to decrease 'shelflife' by 4 if 'outsidetemperature' is greater than 90 can be written in Python as: if outsidetemperature > 90: shelflife -= 4.
Step-by-step explanation:
To write a conditional statement that decreases the variable shelflife by 4 if the variable outsidetemperature is greater than 90, you can use a simple if statement in most programming languages. Here's an example in Python:
if outsidetemperature > 90:
shelflife -= 4
This statement checks whether outsidetemperature is greater than 90. If it is, the statement shelflife -= 4 will execute, effectively reducing the shelflife by 4.