In math, it is common to round to the nearest whole number or the nearest ten to make calculations easier. One tool that can help with rounding is making a number line. No calculator is needed.
Draw a line centering on your number
A number line is a line on which numbers (usually whole numbers) are labeled. They increase as they go from left to right. Put 148 at the center of your number line.
Add numbers before and after your number
Mark the line for each new number and write the numbers in sequence before and after 148. For visual clarity, try to keep the space between the marks even. Because you know you're rounding to the nearest multiple of 10, underline the numbers with a zero in the ones' place.
Figure out which multiple of 10 is closest to your number
On the number line with 148 in the center, it is easy to see that 150 is much closer than 140. This means that 150 is the nearest multiple of 10 to 148. You can also count the number of marks between your number and the nearest 10, assuming one mark for each number. Rounding to the nearest 10 means increasing or decreasing the number by a relatively small amount, so you won't need to change any number by more than five. Digits zero through four round down, and digits five through nine round up.