Distance/speed question

[collegekid]

Ambrose is a bicyclist. He makes a daily ride of 40 miles at a constant speed. If he trains hard, he can increase his speed by 4 miles per hour, which would result in a half-hour decrease in time. Find his new speed.

 

[lookagain]

Make a rate/time/distance table or some other list of information and fill in what you know
to show us before you are assisted.

 

[Denis]

1: (@ a   mph).............40 m................>t hours

2: (@ a+4 mph).............40 m................>t - 1/2 hours

HINT:
1: t = 40/a
2: 40 / (a+4) = 40/a - 1/2