Rounding two-digit numbers to the nearest ten
To round a two-digit number to the nearest ten, we plot them on a number line and check which multiple of ten they are closest to. Numbers ending in 5 (such as 45) are equally far away from the previous and from the next multiple of ten, but mathematicians have decided that these "middle" numbers get rounded UP.
We also look at the case of numbers less than 10 and find out that some of them get rounded to zero.
In the second part, we solve a word problem that asks, "About how many more...?" This means we can use rounded numbers and ESTIMATION, instead of calculating exactly.
See also
Regrouping twice in subtraction
Math Mammoth Grade 3 curriculum