Final answer:
The useful output of a gas boiler with a total input of 1300 joules and an efficiency of 0.6 is 780 joules.
Step-by-step explanation:
To calculate the useful output of a gas boiler with a total input of 1300 joules and an efficiency of 0.6, we use the formula for efficiency: Efficiency = (Useful Output / Total Input). Rearranging the formula to solve for useful output gives us: Useful Output = Efficiency × Total Input.
In this case, the useful output is 0.6 × 1300 joules = 780 joules. Therefore, the gas boiler's useful output is 780 joules.