If dinner for the mathsisons is $45.99 and they want to tip 20% how much will they leave total

Well 45.99 round up to 46 and 20% tip = .20 so
46 • .20 = 9.20 46 + 9.20 = 54.20
so they will leave $54.20

If the Mathsisons were precise enough to do their math, they would have left $55.19 in total.
To get to that final answer, you must first multiply their dinner cost ($45.99) by the tipping price (20%). 45.99 x 20% (or .2) = 9.198. 
Round that answer to the nearest cent. 9.198 ~ 9.20.
This answer will be the tipping price for the Mathsisons' meal.
Add the tipping price to the dinner price to get $55.19 in total.
This price is how much they paid in addition to gratuity.