First, calculate the number of 6 ounces that can be gotten from 42 ounces.
To get this, divide 42 by 6:

It implies that there are 7 of 6 ounces of icing in 42 ounces of icing.
Since it is given that 5 drops of food coloring are used for every 6 ounces, it follows that for seven of 6 ounces, the number of drops of food coloring will be:

The number of drops of food coloring needed for 42 ounces of icing is 35.