If you have a device that has an energy effiency of 0.7 and you put in 150 units of energy how many units of useful energy will you get out of it

The efficiency of 0.7 means 70% of the total output that will be useful.
So in order to answer this type of question you just need to multiply the amount of energy used(150 unit) with the efficiency(0.7)
The calculation will be: 150 x 0.7 = 105 unit
So the amount of useful energy you will get out of it would be: 105 unit