Is a flower having 12 petals trimerous or tetramerous?
Suppose if a flower has 12 petals. Then 12 is a multiple of both 3 and 4. So which category of flower is it? Is it trimerous or tetramerous?

Merosity, in the context of your question, is the number of petals in the corolla.
Refer to &quot;Gray's Botanical Textbook: Structural botany&quot;, on page 176, section 322:

&quot;322. To designate the particular plan, such familiar terms of Latin derivation as binary, ternary, quaternary, quinary, unary, &amp;c., are sometimes employed, denoting that the parts of the flower are in twos, threes, fours, fives, or sixes. More technical and precise terms, equivalent to these, are composed of the Greek numerals prefixed to the word meaning parts or members, as Alonomerous, for the case of a flower of one member of each ; Dimerous, of two, or on the plan of two members of each ; Trimerous, of three, or on the plan of three members ; Tetramerous, of four, or on the quaternary plan ; Pentamerous, of five, or on the quinary plan ; Ilexamerous,$^{[1]}$ of six, or on the plan of six members to each circle. But, in Monocotyledons, so-called hexamerous blossoms are really trimerous, the sixes being double sets of three.
[1] These may be shortly written 1-merous, 2-merous, 3-merous, and so on up to 10-merous (decamerous), 12-merous (dodecamerous), &amp;c.&quot;.

Because counting is done using Greek cardinals a corolla with 12 petals is dodecamerous. One with 4 sets of 3 is trimerous, and one with 3 sets of 4 is tetramerous.