Hello Guest!
Since your week is $5$ days, and there is a $40\%$ chance of sun, we can say that "we expect" $2$ days to have sun. Now, since we have a $20\%$ chance of $4$ inches of rain, we can "expect" $1$ day to have $4$ inches of rain. Now, we have a $40\%$ chance of $10$ inches of rain, hence the last $2$ days we "expect" to have $10$ inches of rain. Now we have $4+10+10=24$ inches of rain from Monday to Friday. This is my take on this problem. Please tell me if this is wrong or does not satisy your problem so I can review and revise. 
But I noticed this 2.5 hours ago.