How Cijferreeksen works
Each question shows a series of numbers that follow each other in a logical way, and you pick the number that completes it from four answers. PiCompany built the part so that it needs only very basic mental arithmetic, at primary-school level. The difficulty is in spotting the rule.
GITP’s own 2023 research shows where that difficulty lies: ChatGPT solved the easy number series but failed once the questions got harder and several operations happened at the same time. Expect harder items to combine two steps, such as doubling and then subtracting.
Cijferreeksen comes last, after Figuurreeksen and Matrixen, so concentration matters. As with the other parts, an untimed explanation and sample questions come first.
How to find the rule
Try the rules in this order, writing as you go:
- Write the difference between each pair of neighbouring numbers underneath the row.
- If those differences are not constant, look at how they change: rising by a fixed amount, doubling, or alternating.
- If adding and subtracting do not fit, try multiplying or dividing, or two operations in turn.
- Read every other number. Two separate series can be woven into one row.
- Check the rule on every number in the row, not just the last few, before you choose.
Worked examples
Series: 4, 7, 13, 25, 49, ? The differences are 3, 6, 12 and 24, doubling each time, so the next difference is 48 and the answer is 97. The same rule written as two operations: double the number, then subtract 1 (4 × 2 − 1 = 7, 49 × 2 − 1 = 97). An option of 98 is what you get if you double and forget to subtract.
Series: 2, 20, 4, 17, 8, 14, ? The differences jump about, which is the sign of two series woven together. The 1st, 3rd and 5th numbers (2, 4, 8) double; the 2nd, 4th and 6th (20, 17, 14) fall by 3. The missing number is 7th, so it belongs to the doubling series: 16.
Tips for the real Cijferreeksen
- Have pen and paper ready. They are the only aids allowed; there is no calculator.
- Spend the first seconds on the differences, not on staring at the row. Written differences show most rules at once.
- Keep the same steady pace you had in the first part. This is the last part of the test.
- If you cannot find the rule, guess from the options that fit best. A blank is marked wrong and a wrong answer is not penalised.
- Do not chase speed. GITP records it but does not score it.
Practise Cijferreeksen on CogniPrep
Our Number Series game draws from our own bank, with four options and the missing number at the end of the row or in the middle, about half each. It adapts over 10 to 15 questions, gives 90 seconds a question with no going back, and ends with a practice percentile converted to GITP’s T-score scale. Feedback tells you if middle gaps or two series woven together cost you more marks.