Consider the following premises. For all y_4, if y_4 were a cashier, then y_4 is drinking water. For all y_4, y_4 will drink water if and only if y_4 will write a letter. For all y_4, y_4 is a cashier. Can we infer the following from them? For all y_4, y_4 will drink water.
 From the fact that for all y_4, y_4 is a cashier we can infer that for all y_4, y_4 is a cashier or y_4 will write a letter. From the fact that for all y_4, y_4 will drink water if and only if y_4 will write a letter we can infer that for all y_4, if y_4 writes a letter, then y_4 is drinking water. Finally, from the fact that for all y_4, if y_4 were a cashier, then y_4 is drinking water, that for all y_4, if y_4 writes a letter, then y_4 is drinking water, and that for all y_4, y_4 is a cashier or y_4 will write a letter we can infer that for all y_4, y_4 will drink water. Therefore, the answer is yes.