The target number is at the top. Tap chips to make exactly that sum: two or three are enough at first, more later on. Tapped a wrong one — tap it again to remove it; “Reset” clears everything. Once the sum is right, press “Check”. Each puzzle is timed — the counter runs at the top: running out or getting the sum wrong counts as an error, and two are allowed per level. Trains mental arithmetic and combination search.