15 men can do a work in 15 days working 8 hours a day. N men can do twice the work in 20 days working 10 hours a day. What is the value of N?
$$\frac{men_{_1}\times\ days_{_1}\times\ time_{_1}}{work_{_1}}\ =\ \frac{men_{_2}\times\ days_{_2}\times\ time_{_2}}{work_{_2}}$$
15 men can do a work in 15 days working 8 hours a day. N men can do twice the work in 20 days working 10 hours a day.
$$\frac{15\times\ 15\times\ 8}{1}\ =\ \frac{N\times20\times10}{2}$$
$$15\times 15\times 8 = N\times100$$
$$1800 = 100N$$
So N = 18
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