These are the ones where you need to find out how they made the middle number (the one in square brackets), using the two numbers on the sides.
So, question 67 for example.
The 86 in the square brackets is made by multiplying the side numbers by 2, and then adding them together; ie (28 x 2) + (15 x 2).
The 94 is made in the same way; ie (12 x 2) + (35 x 2)
So then, using the same method, we need to work out what the middle number should be for the last set. The answer is (28 x 2) + (17 x 2) = 90.
Does this make sense?
o, yes...thank you:)