The table shows the battery life of four different mobile phones: mobile phone battery life phone battery life (hours) a 20 b 25 c 10 d 18 if 8.45% of the battery life of each mobile phone is used in a day by a typical user, for which mobile phone is 0.8 hour of battery life used in a day?

Phone C

Using quick mental math, we can do 8% of 10 to get 0.8, which is close to the value of 0.0845*10. Also fairly quick is moving the decimal point over 1 spot to the right and we go from 0.0845 to 0.845 and that rounds to 0.8 when rounding to one decimal place. Recall that 8.45% = (8.45/100) = 0.0845

If you want to compute out the four results with a calculator, then it would look like this:
0.0845*20 = 1.69
0.0845*25 = 2.1125
0.0845*10 = 0.845
0.0845*18 = 1.521
and this shows that no other result (but choice C) is close to 0.8

So that's why the answer is choice C

  d: 0.7(t + 4 + 2) = t .