Total distance=50=15t+10(4-t) where t is the time travelled at 15mph and 4-t the time travelled at 10mph.
So 50=15t+40-10t, 10=5t, so t=2 hours.
(She rode 15×2=30 miles at 15mph and 10×2=20 miles at 10mph. 30+20=50 miles.)
Let n and n+1 be the numbers.
4n-3(n+1)=4, 4n-3n-3=4, n=7, the smaller number.