What I know

5 hours for push mower 

70 minutes for powerful riding mower

90% is done by p.r.m and rest by p.m

5 hours=300 minutes

70 minutes for 100% 

But we have to find 90%, so,

90%=90/100=0.9

70 X 0.9 = 63

So 90% of 70 is 63. It took Manny 63 minutes to mow 90% of the lawn with his powerful riding mower.

300 minutes for 100% 

But we have to find 10%, so,

10%=10/100=0.1

300 X 0.1 = 30

So 10% of 300 is 30. It took Manny 300 minutes to mow 10% of the lawn with his push mower. 

Now we have to add both of the numbers to get how much time it took Manny Kerr to mow the entire lawn. 

Powerful Riding Mower - 63 

Push Mower -30

63+30=93

Therefore, Manny Kerr took 93 minutes/ 1 hour 33 minutes to mow the entire lawn.