Grace starts with 100 milligrams of a radioactive substance. The amount of the substance decreases by 1/4 each week for a number of weeks, w. She writes the expression 100 (1/4) w to find the amount of radioactive substance remaining after w weeks.

True

Step-by-step explanation:

Each week the amount of radio active material is 100 * (1/4) ^x which is correct.

So at the end of 4 weeks she will have

amount = 100 * (1/4) ^x

amount = 100 * (1/4) ^4

amount = 100 * (1/256)

amount = 0.390625 grams after 4 weeks.