The answer is 188 ! hope this helps!
Answer:
a) Two half lives, b) 
Step-by-step explanation:
a) The polonium-210 has a half life of 138.4 days. Therefore, 1.994 half lives have past.
b) Mass decay is described by the following exponential model:

The time constant for the isotope is:


The mass of the isotope after 276 days is:


32+56*45
Remember PEMDAS, so we have to solve the multiplication part of the expression first....
32+2520
Now we solve for the addition
2552
Answer=2552