Simple arithmetic means you read a sum left to right. You might write it down, you might do it all in that big ol' brain of yours. But still, the term simple arithmetic means you do a sum, left to right as written.

Good news - you've got a bonus at work this month AND you did some overtime.

Your base pay is £2,000. Your bonus is 5% of your base pay and you get £10 per hour overtime and you did 8 hours.

2000 + 2000 x 0.05 + 10 x 8

Enjoy your £1,680