Most of us have little trouble working out how many milliliters are in 2.4 liters of water (it's 2,400). But the same can't be said when we're asked how many minutes are in 2.4 hours (it's 144).

They say it helps children compose and decompose numbers, which is useful in learning addition and subtraction facts. For example, 7 + 5 can be a tricky sum to learn. But a child who can subitize may ...