speed of Michael = 60 mph
distance traveled in 1 hour = speed* time= 60 *1 = 60 miles
marker crossed after 1 hour = 84
so start point of Michael had marker = 84-60 =24
to write linear equation, lets consider a general linear equation y = mx + c
where m is slope and c is constant, y is marker and x is actual distance traveled from starting point
putting y = 84 x = 60 and y =24 and x = 0 in the equation we get
84 = m*60 + c
24 = m*0 + c
on solving,
m = 1 c = 24
so the equation is y = x + 24
distance traveled in 90 minutes after crossing sandy river = 60*90/60 = 90 miles
total distance traveled from starting point = 90+60 = 150
putting x = 150 in y =x+24
y = 174
highway marker he will pass 90 minutes after crossing the sandy river = 174
