You get a strip of numbered cells and a target strip.
Tap a dashed line between two cells to fold there. The shorter side flips over the longer side. If both sides are equal, either gives the same strip.
Cells that land on each other add together.
Match the target using all of your folds. A flipped (reversed) match counts.
Run out of folds without matching and that is a failed attempt. Undo and reset are free. Hints are shown when you share your result.
Worked example
2513→65
Fold in the middle. The 5 lands on the 1 and the 2 lands on the 3, giving 6 and 5.
Six strategy tips
Check the total. Folding never changes the sum, so the start and target add up to the same number. If a plan breaks that, it is wrong.
Count the length budget. A fold removes as many cells as its shorter side. If the strip has 10 cells, the target has 4, and you have 3 folds, the folds must remove 6 cells between them. That limits which creases are possible.
Work backwards from the biggest target number. It is a stack of cells that were folded onto each other. Look for cells in the start strip that add up to it and sit at mirrored distances from one crease.
Fold near the ends early. Folding a short end removes few cells and keeps options open. Big folds in the middle use up the length budget fast.
Use the mirror rule. A fold at one crease pairs the cell just left of it with the cell just right of it, then the next pair outward, and so on. Write the pairs down before you tap.
Remember the flip. The target can match in either direction, so you have two chances to line up your last fold.