The average weight of a group of 51 boys was calculated as 62 kg. It was later discovered that the weight of one of the boys was read as 65 kg. Whereas his actual weight was 45 kg. What is the actual average weight of the group of 51 boys? (rounded off to two digits)

An object travels $20$ meters in $4$ seconds and $15$ meters in the next two seconds. What is the average speed of the object?

$35$ is the average marks obtained by $120$ students. The average marks of successful candidates was $39$, and that of unsuccessful candidates was $15$. What is the number of successful candidates in that examination?

Average of three numbers $\mathrm{B}, \mathrm{C}$ and $\mathrm{D}$ is $1$ more than the average of $\mathrm{A}, \mathrm{B}$ and $\mathrm{C}$. Average of $\mathrm{A}$ and $\mathrm{D}$ is $19.5$.

Quantity $1$: Find $\mathrm{A}$.

Quantity $2$: $21$.