Problem
COM-B1-M01-P017 Pairs with Even Sum
#17
★★★☆☆ Level 3 of 5
How many pairs \((a,b)\), where \(1\le a,b\le20\), have even sum?
The sum is even when both numbers have the same parity.
Among \(1\) to \(20\), there are \(10\) even and \(10\) odd numbers. Pairs with even sum: both even \(10\cdot10\) or both odd \(10\cdot10\). Total: \(200\).
Casework by parity.