Step by step
The question
Question: find the value of ab − 2b when a = 5 and b = 3.
ab − 2b
a = 5, b = 3
1 Put the numbers in
We put 5 in place of a and 3 in place of b.
ab means a times b.
So we write a times sign.
= 5 × 3 − 2 × 3
ab = a × b
2 Times and divide
Times comes before minus.
Times and divide: we go from left to right.
Order: ( ) → x² √ → × ÷ → + −
same kind: left → right
5 times 3 is 15.
= 15 − 2 × 3
Times comes before minus.
2 times 3 is 6.
= 15 − 6
3 Plus and minus
15 minus 6 is 9.
= 9
Careful!
Many people go from left to right and get 39.
But times and divide come before plus and minus.
Careful: not 5 × 3 − 2 × 3 = 39 ✗
So ab − 2b is 9 when a = 5 and b = 3.
Let's look back.
First: put the numbers in.
Then: times and divide.
Last: plus and minus.
Answer
= 9