How a drill works

You start at a number and keep taking away (or adding) the same step. Each answer becomes the next starting point.

100 −7 → 93 −7 → 86 −7 → 79 …

This one is called Serial 7s. Doctors use it as a quick concentration check, which tells you what it trains: holding a number in mind while you work on it. Aim for accuracy first. Speed follows on its own after a few days.

Stop at the ten

Split the step so the first part lands on a round number. Round numbers are easy to work from.

93 − 7 = 93 − 3 − 4 = 90 − 4 = 86

Ask yourself: how far down to the ten? Take that, then take the rest.

Round, then fix

Taking 10 is effortless. So treat 7 as "10, then give 3 back". The same idea works for 8, 9, 17 and 19.

86 − 7 = 86 − 10 + 3 = 79
64 − 19 = 64 − 20 + 1 = 45

Follow the last digit

Taking away 7 always adds 3 to the last digit, wrapping past 9. That gives a cycle you can learn by heart:

Then the tens: if the last digit wrapped past 9, the tens stay the same. Otherwise the tens go down by one.

72 → last digit 2 + 3 = 5, no wrap, tens 7 → 6 → 65
58 → last digit 8 + 3 = 11, wrap to 1, tens stay 5 → 51

Adding steps

The same two moves work in the other direction. Add the round part first, then the rest.

47 + 13 = 47 + 10 + 3 = 60
68 + 17 = 68 + 20 − 3 = 85

Memory mode

In memory mode the number disappears once you answer it, just like the real test. A few habits help:

• Say each answer once, silently, as you type it. Hearing it helps you hold it.
• Keep only one number in mind: the last answer. Forget the older ones.
• If you lose your place, do not guess. Wait two seconds; the number usually comes back.
• Start with steps of 3 before trying 7.

A plan for your first week

About ten minutes a day is enough. Move on when a drill feels easy, not when the day changes.

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