Question 482135
Work done by push lawn mower in 1 min = 1/90

Work done by riding mower in 1 min = 1/45

Work done together by both lawn mower in 1 min = (1/90) + (1/45) = 3/90 = 1/30

So working together it will take to mow the lawn = 1/(1/30) mins

= 30 mins