First let's find how many total ounces it would be by multiplying 16 ounces by the number of mugs (26).
16 × 26 = 416
So she needs 416 ounces of coffee. Now we have to convert ounces to quarts. We need to know that there are 32 ounces per quart. So to find quarts we divide 416 ounces by 32...
416 ÷ 32 = 13 quarts
*FYI- since the mugs hold 16 ounces we can see that 16 is half of 32 so we could have just taken 1/2 of 26 mugs which is 13.
