Question 1003343
Let their rates of working = Ra, Rb, Rc, and Rd
{{{ R[a] = 1/2 }}} ( he makes 1 sub in 2 min )
{{{ R[a] + R[b] = 5/6 }}}
{{{ R[b] + R[c] = 7/12 }}}
{{{ R[c] + R[d] = 9/20 }}}
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Adding the 2nd and 4th equations:
{{{ R[a] + R[b] + R[c] + R[d] = 5/6 + 9/20 }}}
{{{ R[a] + R[b] + R[c] + R[d] = 50/60 + 27/60 }}}
{{{ R[a] + R[b] + R[c] + R[d] = 77/60 }}}
All 4 together can make 77 subs in 1 hour