Question

john is training for a competition in which athletes will run and bike several miles. if john can run a mile in 7 minutes and bike a mile in 4 minutes, he wiil be able to finish the race in 90 minutes. if he can run a mile in 8 minutes and bike a mile in 3 minutes, he will be able to cover the same distance in just 84 minutes.

in the equation below, r represents the number of miles run. and b represents the number of miles biked. 7r + 4b = 90 ...... 8r + 3b = 84 . how many miles will john run during the race ? somebody help please