Practice: compute 245 + 678 + 55 mentally.

978 — pair 245 and 55 first to make 300, then add 678.

245 + 678 + 55 → 245 + 55 = 300 → 300 + 678 = 978.

Addition can be regrouped freely, so look for pairs that land on round numbers before you grind through the terms in order.