Question 916524
x time it takes Robert to do the task alone
{{{1/20 + 1/x = 1/15}}}Multiplying thru by 60x so as all denominators = 1
3x + 60 = 5x
60 = 2x
30 = x, time it takes Robert to do the task alone
...
And...checking
{{{3/60 + 2/60 = 1/15}}}