Practice makes perferct. A saying that also applies to math.
This app offers you more exercises than any exercise book

Subjects:

Counting:
Numbers up to 1 million, larger/smaller, place value table, number line, mean, round, neighboring numbers

Calculate:
Addition, subtraction, multiplication, division, mental arithmetic, gap tasks, written arithmetic, written arithmetic with gaps, arithmetic wall, arithmetic table, arithmetic snake, multiplication table, half-written, counting with times, remainder, dot-before-dash

Geometry: dice counting, dice nets, solids, mirroring,

Time: clock, read time, arithmetic, length of time, days, months, schedule

Money: adding up, subtracting, spending, change, shopping

Units: lengths, weights, liters, conversions, fractions

Data: bar chart, tables, average, probability

Mind games: memorize numbers, sliding puzzles, French solitaire, sudoku, towers of Hanoi, piano, turning puzzles, jigsaw puzzles