Explanation:
It is convenient to regroup the numbers, and adjust two of them to "make a ten".
= 13 + 17 + 19 . . . . rearrange (swap the order of 17 and 19)
= (10 + 3) + (-3 + 20) + 19 . . . . "make a ten" by adding or subtracting a small number. Here, those adjustments cancel—which is the point of doing it.
= 10 + 20 + 19
= 30 + 19
= 49
