It is indeed a correct postfix expression. Visualize it as an input sequence which begins with 10 and the input comes in the order:-
10, 5, +, 60, 6, /, *, 8, -
as soon as 10 is seen it is pushed onto the stack.
after 10 the next input is 5 which is again pushed onto the stack:
The next input is a + operator, whenever a binary operator is seen, the top two values on the stack are popped and the expression formed by the operator and the operand is evaluated: 10+5, the result of this expression( which in this case is 15) will be pushed back onto the stack. Now the stack should look like:
the next inputs 60 and then 6 are similarly pushed onto the stack :
The next operator seen is /, which will be evaluated as: 60/6=10 and 10 will be pushed onto the stack;
Similarly for the * operator: 15*10=150
The next input is 8 which will be pushed onto stack:
Now the final input is a subtraction operator: 150-8=142
This value left on the stack is the evaluation of your postfix expression.
I guess this should have cleared your doubt. If the confusion still persists, feel free to ask for clarification.