miles per hour
(15 and 3/4) miles per 3 hours
remember that
(a+b)/c=(a/c)+(b/c)
and
a and c/d=a+c/d
so
(15 and 3/4)/3=(15/3)+(3/4)/3
remember that dividing by a number is like multiplying by a fraction
15/3=5
(3/4)/3=(3/4) times (1/3)=3/12=1/4
(15/3)+(3/4)/3=5+1/4=5 and 1/4
answer is 5 and 1/4 miles per hour
