Math!

C.

Step-by-step explanation:

This is a multi-step equation, considering that there are three steps.

Step 1: 17 x 0.51.  There are 17 light envelopes, so multiply these two numbers and you will get 8.67.

Step 2: 13 x 0.73.  13 heavy envelopes means to multiply 13 by the cost, which is 0.73.  This should get you 9.49.

Step 3: Add the two numbers to get the total cost.  9.49 + 8.67 = $18.16.

water molecules will move from the side of higher water concentration to the side of lower concentration until both solutions are isotonic. at this point, equilibrium is reached. in an isotonic solution, the flow of water in and out of the cell is happening at the same rate.

hope this < 3

4444444444444 i think