Question 993393
The first boiler uses 1500tonnes/4weeks=375 tonnes/wk
The second uses 2300tonnes/3wks=766.67tonnes/wk
Together:375 tonnes/wk+766.67 tonnes/wk=1141.67 tonnes/wk
.
Coal left when second boiler starts=10000tonnes-3wks(375tonnes/wk) 
Coal left when second boiler starts=10000tone-1125tonnes=8875 tonnes
.
Together the two boilers will burn 8875 tonnes:
8875 tonnes/1141.67 tonnes per week=7.77 weeks
.
ANSWER: The stockpile will be consumed in 7.77 weeks