Question

I know q = ∆ u+ w but work done is negative so answer should be 20j but answer is 40j​

Answer 1

Answer:

When work is done by system it means energy is going out of system or decreasing.

Increase in internal energy means that work was done on system which converted to internal energy.

Now back to your question. Assume initially your system has zero energy. We gave him some.

After it works 10 j or we take 10 j away from him still 30 j left. So how much we gave him . 40 j

You can use formula but in question

which energy is positive or negative be careful