How would I go about designing an If-Then-Else statement or a flowchart with a dual alternative decision structure) that assigns 0 to the variable b if the variable a is less than 10? Otherwise, it should assign 99 to the variable b.

Not sure what language you're into, but in general the logic would be like this:

if(a<10)
then b = 0;
else b = 99;
endif

if temperature is <0 Then

Else if variable is > 10 then
Display "Variable is A
Else if variable is