For every two 20cm by 25cm notebooks Ann orders, she orders one 12 cm notebooks. The order totals 48 items. How many of larger book did she order?

So,

For every 2 large notebooks Ann orders, she orders 1 small notebook.

This can be stated as a ratio: 2:1

Add up the units to find the value of each unit.

2 + 1 = 3

Divide 48 by 3 to get the unit value.

48/3 = 16

The unit value is 16.

The number of large notebooks Ann orders is 2 (16) : 16

This is a 2:1 ratio.

Therefore, Ann ordered 32 large notebooks and 16 small notebooks.