ok so x + y = n (where n is the number of the original order, x is the part that went to the store and y is the number in the warehouse.)
25 = 20% of x, so x = (25 divided by .2) = 125 notebooks went to the store.
125 = 25% of total n (because 75% went into warehouse and 100-75 = 25).
so 125 divided by .25 = 500 notebooks. total.
and (500-125 = 375 notebooks into the warehouse.)