A SIKAKU puzzle consists of a square or rectangular grid in which some of the cells contain a one or two digit number. To complete the puzzle, a Solver must divide the puzzle into a number of rectangles so that each rectangle contains exactly one of the numbers, and that number must be equal to the area of the rectangle which contains it. Each puzzle has a unique solution which does no require any guesswork to achieve.
The 25 must be 5×5, and there's only one way to let it be. This forces the 5 above it to stretch to the corner as no other number can reach. It also squeezes the 7, 4, and 10 below it and lets them be resolved.
Various reachability deductions in the bottom-left corner. Only the 8 can get to the corner, so it must stretch all the way there. Then only the 4 can get to the square above the 8, so we can resolve it. Finally only the 12 can stretch to R11C1, so it does so.