The injury would be fully healed after 3*5=<<3*5=15>>15 days
So he would start exercising again in 15+3=<<15+3=18>>18 days
He would then wait for 3*7=<<3*7=21>>21 more days to start lifting heavy again
That means he needs to wait 18+21=<<18+21=39>>39 days
#### 39